dp（i，j）表示扔i次j个硬币朝上的概率，然后就很好想了。。。

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;
const int maxn = 500;
int n, k;
double dp[maxn][maxn];
int main() {
    scanf("%d %d", &n, &k);
    for (int i = 0; i <= k; i++) {
        for (int j = 0; j <= n; j++) {
            dp[i][j] = 0.0;
        }
    }
    dp[0][0] = 1.0;
    for (int i = 0; i < k; i++) {
        for (int j = 0; j <= n; j++) {
            if (j != n) {
                dp[i+1][j] += dp[i][j] / 2;
                dp[i+1][j+1] += dp[i][j] / 2;
            }
            else {
                dp[i+1][j-1] += dp[i][j] / 2;
                dp[i+1][j] += dp[i][j] / 2;
            }
        }
    }
    double ans = 0;
    for (int i = 0; i <= n; i++) {
        ans += dp[k][i]*i;
    }
    printf("%.8lf\n", ans);
    return 0;
}