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33 lines
543 B
Python
33 lines
543 B
Python
def problem_p02768(input_data):
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n, a, b = list(map(int, input_data.split()))
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mod = 10**9 + 7
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a = min(a, n - a)
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b = min(b, n - b)
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m = max(a, b)
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X = [0] * (m + 1)
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Y = [0] * (m + 1)
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X[0] = 1
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X[1] = n
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Y[0] = Y[1] = 1
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for i in range(2, m + 1):
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X[i] = X[i - 1] * (n - i + 1) % mod
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Y[i] = Y[i - 1] * i % mod
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ans = (
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((pow(2, n, mod) - 1) % mod - X[a] * pow(Y[a], mod - 2, mod) % mod) % mod
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- X[b] * pow(Y[b], mod - 2, mod) % mod
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) % mod
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return ans
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