11 lines
210 B
Python
11 lines
210 B
Python
if __name__ == "__main__":
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N = int(input())
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A = list(map(int, input().split()))
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AS = sum(A)
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S = 0
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for _ in range(N - 1):
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r = A.pop()
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AS -= r
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S += r * AS
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print(S)
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