#include <iostream>
using namespace std;
int main() {
int n;
double sum = 0;
cin >> n;
double x1, x2, x3;
cin >> x2 >> x3;
for (int i = 0; i < n-2; i++ ){
x1 = x2;
x2 = x3;
cin >> x3;
sum += (x1 + x2 + x3) * x2;
}
cout << "sum" << " = " << sum;
return 0;
}
I2luY2x1ZGUgPGlvc3RyZWFtPgp1c2luZyBuYW1lc3BhY2Ugc3RkOwoKaW50IG1haW4oKSB7CglpbnQgbjsKCWRvdWJsZSBzdW0gPSAwOwoJY2luID4+IG47Cglkb3VibGUgeDEsIHgyLCB4MzsKCWNpbiA+PiB4MiA+PiB4MzsKCQoJZm9yIChpbnQgaSA9IDA7IGkgPCBuLTI7IGkrKyApewoJCXgxID0geDI7CgkJeDIgPSB4MzsKCQljaW4gPj4geDM7CgkJc3VtICs9ICh4MSArIHgyICsgeDMpICogeDI7Cgl9CgkKCWNvdXQgPDwgInN1bSIgPDwgIiA9ICIgPDwgc3VtOwoJCglyZXR1cm4gMDsKfQ==