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zc
2019-12-21 19:47:00
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#include<bits/stdc++.h>
using namespace std;
#define N 10011
int n;
int main(){
    double ans=0,t=0,x;
    scanf("%d",&n);
    while(n--){
        scanf("%lf",&x);
        ans=ans+x*(2*t+1);
        t=(t+1)*x;
    }
    printf("%.10f\n", ans);
}
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