14 lines
258 B
Python
14 lines
258 B
Python
def solve(N, K):
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Kp = (K * (K + 1)) // 2
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if N < Kp:
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return -1
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else:
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N -= Kp
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r = N % K
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return sum((K - 1, 1 if r > 0 else 0))
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if __name__ == '__main__':
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N, K = map(int, input().split())
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print(solve(N, K))
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