#include <bits/stdc++.h>
using namespace std;
struct gam {
  int o;
  long long k;
  int nez;
  long long bd;
  long long kd;
  gam() {
    o = 0;
    k = 0;
    nez = 0;
    bd = 0;
    kd = 0;
  }
};
bool mycomp(gam i, gam j) { return (i.k < j.k); }
int main() {
  long long n;
  long long r;
  long long tr;
  long long sum = 0;
  cin >> n;
  cin >> r;
  cin >> tr;
  gam* a = new gam[n + 1];
  a[0].k = 0;
  for (int i = 1; i <= n; i++) {
    cin >> a[i].o;
    cin >> a[i].k;
    a[i].nez = r - a[i].o;
    sum = sum + a[i].o;
  }
  sort(a + 1, a + n + 1, mycomp);
  long long nug = tr * n - sum;
  if (nug > 0) {
    int cur;
    for (int i = 1; i <= n; i++) {
      a[i].bd = a[i - 1].bd + a[i].nez;
      a[i].kd = a[i - 1].kd + a[i].k * a[i].nez;
      if (a[i].bd >= nug) {
        cur = i;
        break;
      }
    }
    long long enug = nug - a[cur - 1].bd;
    long long rez = a[cur - 1].kd + enug * a[cur].k;
    cout << rez;
  } else
    cout << 0;
  return 0;
}
