#include<iostream>

#include<cstdio>

#include<cstring>

using namespace std;

const int MAXN = ;

inline int rd(){

    int x=,f=;char ch=getchar();

    while(!isdigit(ch)) {f=ch=='-'?:;ch=getchar();}

    while(isdigit(ch))  {x=(x<<)+(x<<)+ch-'';ch=getchar();}

    return f?x:-x;

}

int n;

double f[MAXN],a1[MAXN],a2[MAXN],p[MAXN];

int main(){

    cin>>n;

    for(int i=;i<=n;i++) scanf("%lf",&p[i]);

    for(int i=;i<=n;i++){

        a1[i]=(a1[i-]+)*p[i];

        a2[i]=(a2[i-]+*a1[i-]+)*p[i];

        f[i]=f[i-]+(*a1[i-]+*a2[i-]+)*p[i];

    }

    printf("%.1lf",f[n]);

    return ;

}