58 lines
1.2 KiB
Python
58 lines
1.2 KiB
Python
import sys
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input = sys.stdin.readline
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n = int(input())
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arr = list(map(int, input().split()))
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if(n==1):
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print('A')
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elif(n==2):
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print('A')
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elif(arr[1]==arr[0]):
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flag = False
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for i in range(1, n):
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if(arr[i]!=arr[0]):
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flag = True
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if flag:
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print('B')
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else:
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print(arr[0])
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elif(n>2 and arr[2]-arr[1] == arr[1]-arr[0]):
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d = arr[1]-arr[0]
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flag = False
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for i in range(1, n):
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if(arr[i]-arr[i-1]!=d):
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flag = True
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if flag:
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print('B')
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else:
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print(arr[n-1]+d)
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elif(n>2 and arr[2]//arr[1] == arr[1]//arr[0]):
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d = arr[1]//arr[0]
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if(arr[1]/arr[0] < 1 and arr[1]*arr[0]<0):
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print('B')
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exit()
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flag = False
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for i in range(1, n):
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if(arr[i]//arr[i-1]!=d):
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flag = True
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if flag:
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print('B')
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else:
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print(arr[n-1]*d)
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else:
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if (arr[1] - arr[0]) == 0:
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print('B')
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exit()
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a = (arr[2] - arr[1]) // (arr[1] - arr[0])
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b = (arr[2] + arr[1] - (arr[1] + arr[0]) * a) // 2
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flag = False
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for i in range(1, n):
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if arr[i] != arr[i - 1] * a + b:
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flag = True
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if flag:
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print('B')
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else:
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print(arr[n - 1] * a + b)
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