#include <iostream>
using namespace std;
int main()
{
int n;
cin>>n;
double x[n];
for (int i=0; i<n; i++)//вводим массив
cin>>x[i];
double p=1;
for (int i=0; i<n-2; i++)//подсчитываем необходимое произведение
{
p*=(x[i]+2*x[i+1]+x[i+2]);
}
cout<<p;
return 0;
}
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