#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 ;

 }