#include <iostream>

using namespace std;

int main()
{
    int n, d = 0;
    cin >> n;
    if (n % 2 == 1)
    {
        cout << 0 << endl;
        return 0;
    }
    n /= 2;
    for (int i = 0; i < n; i++)
    {
        int a, b;
        cin >> a >> b;
        d += a * b;
    }
    cout << d << endl;
    return 0;
}