#include<iostream>
#include<cmath>
using namespace std;
long long Coe[110], Exp[110], n, x, ans = 0;
int main(){
    ios::sync_with_stdio(false);
    cin >> n;
    for(int i = 1; i <= n; i++)cin >> Coe[i] >> Exp[i];
    cin >> x;
    for(int i = 1; i <= n; i++)ans += Coe[i]*pow(x, Exp[i]);
    cout << ans;
}