#include <bits/stdc++.h>
using namespace std;
int main() {
  unsigned long long int n, r, m;
  cin >> n >> r >> m;
  unsigned long long int a[n];
  unsigned long long int b[n];
  unsigned long long int sum = 0;
  pair<unsigned long long int, unsigned long long int> p[n];
  for (unsigned long long int i = 0; i < n; i++) {
    cin >> a[i] >> b[i];
    sum = sum + a[i];
    p[i].first = b[i];
    p[i].second = a[i];
  }
  if (sum / n >= m) {
    cout << "0" << endl;
  } else {
    unsigned long long int rest = n * m - sum;
    sort(p, p + n);
    unsigned long long int q = 0;
    unsigned long long int w = 0;
    while (rest > 0 && w < n) {
      if (p[w].second < r) {
        if (rest > (r - p[w].second)) {
          q = q + p[w].first * (r - p[w].second);
          rest = rest - (r - p[w].second);
          p[w].second = r;
          w++;
        } else {
          q = q + p[w].first * rest;
          rest = 0;
        }
      } else {
        w++;
      }
    }
    printf("%lld", q);
  }
}
