diff --git a/백준/1850.py b/백준/1850.py
new file mode 100644
index 0000000..17fb5a4
--- /dev/null
+++ b/백준/1850.py
@@ -0,0 +1,18 @@
+import math
+import sys
+
+input = sys.stdin.readline
+
+A, B = map(int, input().split())
+
+def GCD(a, b):
+    if b==0:
+        return a
+    else:
+        return GCD(b, a%b)
+
+result = GCD(A, B)
+
+while result > 0 :
+    print(1, end='')
+    result -= 1
\ No newline at end of file
diff --git a/백준/1865.py b/백준/1865.py
new file mode 100644
index 0000000..f4b591c
--- /dev/null
+++ b/백준/1865.py
@@ -0,0 +1,46 @@
+import sys
+input = sys.stdin.readline
+
+#문제는 음수사이클이 존재하냐? 고 묻는 전형적인 벨만포드 문제
+INF = sys.maxsize
+tc = int(input())
+for _ in range(tc):
+    n, m, w = map(int, input().split()) # 지점의 수, 도로의 개수, 웜홀의 개수
+    graph = [[] for _ in range(n+1)]
+    value = False
+    for _ in range(m):
+        s, e, t = map(int, input().split()) # s,e는 연결된 지점번호 t는 가중치
+        graph[s].append((e, t))
+        graph[e].append((s, t))
+
+    for _ in range(w):
+        s, e, t = map(int, input().split()) # s,e는 연결된 지점번호 t는 감소가중치
+        graph[s].append((e, -t))
+    
+    dist = [INF for _ in range(n+1)]
+    dist[0] = 0
+    for i in range(1, n+1):
+        graph[0].append((i, 0))
+
+    for i in range(1, n+1):
+        for current_node in range(0, n+1):
+            if dist[current_node] == INF:
+                continue
+            for next_node, weight in graph[current_node]:
+                if dist[current_node] + weight < dist[next_node]:
+                    dist[next_node] = dist[current_node] + weight
+    for current_node in range(0, n+1):
+        if dist[current_node] == INF:
+            continue
+        for next_node, weight in graph[current_node]:
+            if dist[current_node] + weight < dist[next_node]:
+                value = True
+                break
+        if value:
+            break
+
+    if value:
+        print('YES')
+    else:
+        print('NO')
+
diff --git a/백준/1915.py b/백준/1915.py
new file mode 100644
index 0000000..88df2c9
--- /dev/null
+++ b/백준/1915.py
@@ -0,0 +1,20 @@
+import sys
+
+input = sys.stdin.readline
+
+m, n = map(int, input().split())
+arr = []
+detect = False
+for i in range(m):
+    arr.append(list(map(int, input().rstrip())))
+
+for x in range(1, m):
+    for y in range(1, n):
+        if(arr[x-1][y-1] and arr[x][y-1] and arr[x-1][y] and arr[x][y]):
+            arr[x][y] = min(arr[x-1][y-1], arr[x][y-1], arr[x-1][y])  + 1
+k=max(map(max, arr))
+
+if(k==0):
+    print(k)
+else:
+    print(k*k)
\ No newline at end of file
diff --git a/백준/1916.py b/백준/1916.py
new file mode 100644
index 0000000..fd00133
--- /dev/null
+++ b/백준/1916.py
@@ -0,0 +1,34 @@
+import sys
+from queue import PriorityQueue
+
+input = sys.stdin.readline
+
+n = int(input()) # 도시 개수
+m = int(input()) # 버스 개수
+dist = [sys.maxsize] * (n+1)
+
+myList = [[] for i in range(n+1)]
+visited = [False] * (n+1)
+
+for i in range(m):
+    s, e, w = map(int, input().split())
+    myList[s].append((e,w))
+
+start, end = map(int, input().split()) #시작 도시, 종료 도시
+
+def dijkstra(start, end):
+    queue = PriorityQueue()
+    queue.put((0, start))
+    dist[start]  = 0
+    while (queue.qsize()>0):
+        nowNode = queue.get()
+        now = nowNode[1]
+        if not visited[now]:
+            visited[now] = True
+            for n in myList[now]:
+                if not visited[n[0]] and dist[n[0]] > dist[now]+n[1]:
+                    dist[n[0]] = dist[now]+n[1]
+                    queue.put((dist[n[0]], n[0]))
+    return dist[end]
+
+print(dijkstra(start, end))
\ No newline at end of file
diff --git a/백준/1918.py b/백준/1918.py
new file mode 100644
index 0000000..e344871
--- /dev/null
+++ b/백준/1918.py
@@ -0,0 +1,29 @@
+import sys
+
+input = sys.stdin.readline
+
+result = [] #숫자들 스택
+calc_stack = [] #연산기호 스택
+
+arr = list(map(str, input().strip()))
+
+precedence = {'+' : 1, '-': 1, '*' : 2, '/' : 2}
+
+for index in arr:
+    if index.isalpha():
+        result.append(index)
+    elif index == '(':
+        calc_stack.append(index)
+    elif index == ')':
+        while calc_stack and calc_stack[-1]!='(':
+            result.append(calc_stack.pop())
+        calc_stack.pop()
+    else:
+        while calc_stack and calc_stack[-1]!='(' and precedence[index] <= precedence[calc_stack[-1]]  :
+            result.append(calc_stack.pop())
+        calc_stack.append(index)
+
+while calc_stack:
+    result.append(calc_stack.pop())
+
+print(''.join(result))
\ No newline at end of file