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60 lines
1.2 KiB
C++
60 lines
1.2 KiB
C++
#include <iostream>
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#include <cmath>
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#include <iomanip>
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using std::cin;
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using std::cout;
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const char endl = '\n';
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const int N = 55;
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int n, m, a[N];
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long long c[N][N];
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double f[N][N][N], sum, ans;
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int main() {
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std::ios::sync_with_stdio(false);
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cin.tie(nullptr);
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c[0][0] = 1;
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for (int i = 1; i < N; ++i) {
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c[i][0] = 1;
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for (int j = 1; j <= i; ++j) {
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c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
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}
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}
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cin >> n >> m;
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for (int i = 1; i <= m; i++) {
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cin >> a[i];
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}
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f[0][0][0] = 1;
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for (int i = 1; i <= m; i++) {
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for (int j = 0; j <= n; j++) {
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for (int k = 0; k <= j; k++) {
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for (int l = 0; l <= n - j; l++) {
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int p = std::max(static_cast<int>(std::ceil(static_cast<double>(l) / a[i])), k);
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f[i][j + l][p] += f[i - 1][j][k] * c[n - j][l];
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}
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}
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}
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}
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for (int i = 0; i <= n; i++) {
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sum += f[m][n][i];
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}
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for (int i = 1; i <= n; i++) {
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ans += static_cast<double>(i) * f[m][n][i];
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}
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cout << std::fixed << std::setprecision(20) << ans / sum << endl;
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return 0;
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}
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