#include <bits/stdc++.h>
using namespace std;
void swap(int& a, int& b) {
  int tmp;
  tmp = a;
  a = b;
  b = tmp;
}
void QuickSort(int l, int h, int a[], int b[]) {
  int i = l, j = h;
  int pivot = a[l + rand() % (h - l + 1)];
  while (i <= j) {
    while (a[i] < pivot) i++;
    while (a[j] > pivot) j--;
    if (i <= j) {
      swap(a[i], a[j]);
      swap(b[i], b[j]);
      i++;
      j--;
    }
  }
  if (l < j) QuickSort(l, j, a, b);
  if (i < h) QuickSort(i, h, a, b);
}
int main() {
  int n, r, avg;
  long long s = 0;
  cin >> n >> r >> avg;
  int b[n + 1], a[n + 1];
  for (int i = 1; i <= n; i++) {
    cin >> a[i] >> b[i];
    s += (long long)a[i];
  }
  long long d = (long long)n * avg - s;
  if (d <= 0) {
    cout << 0;
    return 0;
  }
  QuickSort(1, n, b, a);
  long long ans = 0;
  for (int i = 1; i <= n; i++) {
    if (d >= r - a[i]) {
      ans += (long long)(r - a[i]) * b[i];
      d -= r - a[i];
      if (!d) break;
    } else {
      ans += (long long)d * b[i];
      break;
    }
  }
  cout << ans;
}
