#include <bits/stdc++.h>
using namespace std;
int GCD(int x, int y) { return !y ? x : GCD(y, x % y); }
int LCM(int x, int y) { return x * y / GCD(x, y); }
long long ceilDivision(long long dividend, long long divisor) {
  return (dividend + divisor - 1) / divisor;
}
bool check(pair<long long, int> right, pair<long long, int> left) {
  return ((left.second > right.second) ||
          (left.second == right.second && left.first > right.first));
}
int main() {
  long long r, avg, sum = 0, target, counter = 0;
  int n, pointer = 0;
  cin >> n >> r >> avg;
  vector<pair<long long, int> > arr(n);
  target = avg * n;
  for (int i = 0; i < n; i++)
    cin >> arr[i].first >> arr[i].second, sum += arr[i].first;
  if (sum < target) {
    sort(arr.begin(), arr.end(), check);
    target -= sum;
    while (target) {
      if (arr[pointer].first < r)
        counter += (min(target, r - arr[pointer].first) * arr[pointer].second),
            target -= min(target, r - arr[pointer].first);
      pointer++;
    }
  }
  cout << counter << endl;
  return 0;
}
