#include <bits/stdc++.h>
using namespace std;
int read() {
  int q = 0;
  char ch = ' ';
  while (ch < '0' || ch > '9') ch = getchar();
  while (ch >= '0' && ch <= '9') q = q * 10 + ch - '0', ch = getchar();
  return q;
}
const int N = 100005;
int n, r;
long long orz, ans;
struct node {
  int a, b;
} t[N];
bool cmp(node x, node y) { return x.b < y.b; }
int main() {
  n = read(), r = read(), orz = 1LL * read() * n;
  for (register int i = 1; i <= n; ++i)
    t[i].a = read(), t[i].b = read(), orz -= (long long)t[i].a;
  if (orz <= 0) {
    puts("0");
    return 0;
  }
  sort(t + 1, t + 1 + n, cmp);
  for (register int i = 1; i <= n; ++i) {
    if (orz <= (long long)(r - t[i].a)) {
      ans += 1LL * t[i].b * orz;
      break;
    } else
      ans += 1LL * t[i].b * (r - t[i].a), orz -= (long long)(r - t[i].a);
  }
  printf("%lld\n", ans);
  return 0;
}
