#include<bits\stdc++.h>usingnamespace std;constint N =100001;double p[N];double dp[N];intmain(){int n;
cin >> n;double ans =0;for(int i =1; i <= n ; i++){
cin >> p[i];
ans += p[i];}for(int i =2; i <= n ; i++){
dp[i]=(dp[i-1]+p[i-1])*p[i];
ans += dp[i]*2;}
cout <<setiosflags(ios::fixed)<<setprecision(15)<< ans << endl;return0;}