#include <bits/stdc++.h>
using namespace std;

template<class T> bool chmax(T &a, const T &x) { return x > a ? a = x, true : false; }

int main() {
  int N, K;
  long long P;
  cin >> N >> P >> K;

  constexpr long long inf = 1e18;
  vector<long long> dp(K+1, -inf);
  dp[0] = P;

  while (N--) {
    int T, B;
    cin >> T >> B;

    vector<long long> _dp = dp;
    for (int k = 1; k <= K; ++k) {
      chmax(_dp[k], dp[k-1] * T + B);
      if (_dp[k] > inf) {
        cout << -1 << endl;
        return 0;
      }
    }
    dp = move(_dp);
  }
  cout << dp[K] << endl;
  return 0;
}