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题目链接

复制代码

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
inline ll read(){
    int x=0,f=1;char ch=getchar();
    while(ch>'9'||ch<'0'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}

/***********************************************************/

const int maxn = 1e6+7;
double dp[maxn][3];

int main(){
    int n;
    n = read();
    for(int i = 1;i <= n;i++){
        double x;
        scanf("%lf", &x);
        dp[i][0] = (dp[i-1][0] + 1)*x;
        dp[i][1] = (dp[i-1][1] + 2*dp[i-1][0] + 1) * x;
        dp[i][2] = dp[i-1][2] + (3*dp[i-1][1] + 3*dp[i-1][0] + 1) * x;
    }
    printf("%.1lf\n", dp[n][2]);
    return 0;
}