OSU!

题目链接

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

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

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

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