#include <bits/stdc++.h>
using namespace std;
unsigned long long mod = 1e9 + 7;
unsigned long long power(unsigned long long x, unsigned long long y) {
  unsigned long long res = 1;
  x = x % mod;
  if (x == 0) return 0;
  while (y > 0) {
    if (y & 1) res = (res * x) % mod;
    y = y >> 1;
    x = (x * x) % mod;
  }
  return res;
}
unsigned long long ad(unsigned long long a, unsigned long long b) {
  return ((a % mod + b % mod) % mod);
}
unsigned long long sub(unsigned long long a, unsigned long long b) {
  return ((a % mod - b % mod + mod) % mod);
}
unsigned long long prime[168] = {
    2,   3,   5,   7,   11,  13,  17,  19,  23,  29,  31,  37,  41,  43,
    47,  53,  59,  61,  67,  71,  73,  79,  83,  89,  97,  101, 103, 107,
    109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181,
    191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263,
    269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349,
    353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433,
    439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521,
    523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613,
    617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701,
    709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773, 787, 797, 809,
    811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887,
    907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997};
unsigned long long fact(unsigned long long n) {
  if (n <= 1) return 1;
  return (fact(n - 1) * (n));
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  unsigned long long t1 = 1;
  while (t1--) {
    unsigned long long n, r, a, x, y, s = 0;
    cin >> n >> r >> a;
    vector<pair<unsigned long long, unsigned long long> > v;
    for (unsigned long long i = 0; i < n; i++) {
      cin >> x >> y;
      v.push_back(make_pair(y, x));
      s += x;
    }
    if ((s / n) >= a) return cout << 0, 0;
    x = n * a - s;
    unsigned long long ans = 0, i = 0;
    sort(v.begin(), v.end());
    while (x > 0) {
      unsigned long long u = min(x, r - v[i].second);
      ans += u * v[i].first;
      x -= u;
      i++;
    }
    cout << ans;
  }
  return 0;
}
