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#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int maxn = 100005;
int main(){
int n;
double ans = 0,count = 0,list[maxn];
cin >> n;
for(int i = 0;i < n;i++){
cin >> list[i];
ans += list[i] * (n - count) * (i + 1);
count ++;
}
printf("%.2f",ans);
return 0;
}
