#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <cmath>using namespace std;typedef long long ll;const int N=1e5+5;inline int read(){    char c=getchar();int x=0,f=1;    while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1;c=getchar();}    while(c>=‘0‘&&c<=‘9‘){x=x*10+c-‘0‘;c=getchar();}    return x*f;}int n;double a,f[N][3];int main(){    freopen("in","r",stdin);    n=read();    for(int i=1;i<=n;i++){        scanf("%lf",&a);        f[i][0]=(f[i-1][0]+1)*a;        f[i][1]=(f[i-1][1]+2*f[i-1][0]+1)*a;        f[i][2]=f[i-1][2]+(3*f[i-1][1]+3*f[i-1][0]+1)*a;    }    printf("%.1lf",f[n][2]);}