#include <bits/stdc++.h>
using namespace std;
long long mod = 1;
struct infrm {
 public:
  long long x, y;
} var[100005];
long long n, m;
long long sum;
int ar[100005], br[100005];
long long r;
bool cmp(infrm a, infrm b) { return a.y < b.y; }
bool cmp1(infrm a, infrm b) { return a.x < b.x; }
int main() {
  int i, j, t, tc = 1, a, b;
  scanf("%lld %lld %lld", &n, &m, &r);
  for (i = 0; i < n; i++) {
    scanf("%lld %lld", &var[i].x, &var[i].y);
    sum += var[i].x;
  }
  if ((sum / n) >= r) {
    printf("0");
    return 0;
  }
  stable_sort(var, var + n, cmp1);
  stable_sort(var, var + n, cmp);
  long long rem = (long long)(r * n) - (sum);
  sum = 0;
  long long pin;
  for (i = 0; i < n; i++) {
    long long now =
        (long long)min(rem * var[i].y, (long long)(m - var[i].x) * var[i].y);
    sum += now;
    rem -= min((long long)rem, (long long)(m - var[i].x));
    if (rem <= 0) break;
  }
  cout << sum << endl;
  return 0;
}
