#include<bits/stdc++.h>
using namespace std;
int main(){
    int n, s, k, p, q, ans = 0;
    cin >> n >> s >> k;
    for(int i = 1; i <= n; i++) cin >> p >> q, ans += p * q;
    if(ans < s) ans += k;
    cout << ans;
    return 0;
}