#include <bits/stdc++.h>
using namespace std;
template <class T>
T _abs(T n) {
  return (n < 0 ? -n : n);
}
template <class T>
T sq(T x) {
  return x * x;
}
template <class T>
T gcd(T a, T b) {
  return (b != 0 ? gcd<T>(b, a % b) : a);
}
template <class T>
T lcm(T a, T b) {
  return (a / gcd<T>(a, b) * b);
}
template <class T>
T _max(T a, T b) {
  return (!(a < b) ? a : b);
}
template <class T>
T _min(T a, T b) {
  return (a < b ? a : b);
}
const int INF = 0x7f7f7f7f;
template <class T>
void deb(T e) {
  cout << e << endl;
}
template <class T1, class T2>
void deb(T1 e1, T2 e2) {
  cout << e1 << "\t" << e2 << endl;
}
template <class T1, class T2, class T3>
void deb(T1 e1, T2 e2, T3 e3) {
  cout << e1 << "\t" << e2 << "\t" << e3 << endl;
}
template <class T1, class T2, class T3, class T4>
void deb(T1 e1, T2 e2, T3 e3, T4 e4) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << endl;
}
template <class T1, class T2, class T3, class T4, class T5>
void deb(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5) {
  cout << e1 << "\t" << e2 << "\t" << e3 << "\t" << e4 << "\t" << e5 << endl;
}
template <class T>
void deb(vector<T> e) {
  int i;
  for (i = 0; i < (int)e.size(); i++) cout << e[i] << " ";
  cout << endl;
}
template <class T>
void deb(vector<T> e, int n) {
  int i;
  for (i = 0; i < n; i++) cout << e[i] << " ";
  cout << endl;
}
template <class T>
void deb(vector<basic_string<T> > e) {
  int i, j;
  for (i = 0; i < (int)e.size(); i++) {
    for (j = 0; j < (int)e[i].size(); j++) cout << e[i][j];
    cout << endl;
  }
  cout << endl;
}
template <class T>
void deb(vector<basic_string<T> > e, int row, int col) {
  int i, j;
  for (i = 0; i < row; i++) {
    for (j = 0; j < col; j++) cout << e[i][j];
    cout << endl;
  }
  cout << endl;
}
template <class T>
void deb(vector<vector<T> > e, int row, int col) {
  int i, j;
  for (i = 0; i < row; i++) {
    for (j = 0; j < col; j++) cout << e[i][j] << "\t";
    cout << endl;
  }
  cout << endl;
}
template <class T>
void deb(T e[100005][100005], int row, int col) {
  int i, j;
  for (i = 0; i < row; i++) {
    for (j = 0; j < col; j++) cout << e[i][j] << " ";
    cout << endl;
  }
}
template <class T>
void deb(T e[], int row) {
  int i;
  for (i = 0; i < row; i++) cout << e[i] << " ";
  cout << endl;
}
template <class T>
inline bool isPrime(T n) {
  if (n <= 1) return false;
  for (T i = 2; i * i <= n; i++)
    if (n % i == 0) return false;
  return true;
}
template <class T>
string toString(T n) {
  ostringstream oss;
  oss << n;
  oss.flush();
  return oss.str();
}
int toInt(string s) {
  int r = 0;
  istringstream sin(s);
  sin >> r;
  return r;
}
bool IsVowel(char ch) {
  ch = tolower(ch);
  if (ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u')
    return true;
  return false;
}
bool isUpper(char c) { return c >= 'A' && c <= 'Z'; }
bool isLower(char c) { return c >= 'a' && c <= 'z'; }
long long int Pow(long long int B, long long int P) {
  long long int R = 1;
  while (P > 0) {
    if (P % 2 == 1) R = (R * B);
    P /= 2;
    B = (B * B);
  }
  return R;
}
int BigMod(long long int B, long long int P, long long int M) {
  long long int R = 1;
  while (P > 0) {
    if (P % 2 == 1) {
      R = (R * B) % M;
    }
    P /= 2;
    B = (B * B) % M;
  }
  return (int)R;
}
void binprint(long long int mask, long long int n) {
  long long int i;
  string s = "";
  do {
    s += (mask % 2 + '0');
    mask /= 2;
  } while (mask);
  reverse(s.begin(), s.end());
  s = string(max(n - (int)s.size(), 0LL), '0') + s;
  for (i = (int)s.size() - n; i < (int)s.size(); i++) printf("%c", s[i]);
  printf("\n");
}
void ASCII_Chart() {
  int i, j, k;
  printf("ASCII Chart:(30-129)\n");
  for (i = 30; i < 50; i++) {
    for (j = 0; j < 5; j++) {
      k = i + j * 20;
      printf("%3d---> '%c'   ", k, k);
    }
    printf("\n");
  }
}
vector<pair<long long int, long long int> > arr;
int main() {
  long long int i, j, k, u, v, w, x, y, a, b, z, t = 0, tcase, res = 0, n, r,
                                                 avg;
  while (cin >> n >> r >> avg) {
    long long int need = 0, have = 0;
    need = n * avg;
    arr.clear();
    for (i = 0; i < n; i++) {
      cin >> a >> b;
      arr.push_back(make_pair(b, a));
      have += a;
    }
    if (have >= need) {
      cout << 0 << endl;
    } else {
      res = 0;
      sort(arr.begin(), arr.end());
      for (i = 0; i < (int)arr.size(); i++) {
        if (need > have && arr[i].second < r) {
          x = min((need - have), abs(arr[i].second - r));
          res += (x * arr[i].first);
          have += x;
        }
      }
      cout << res << endl;
    }
  }
  return 0;
}
