/* -*- coding: utf-8 -*-
 *
 * 2693.cc:  No.2693 Sword - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 1000;
const int MAX_K = 1000;
const long long MAX_D = 1000000000000000000LL;

/* typedef */

typedef long long ll;

/* global variables */

int ts[MAX_N], bs[MAX_N];
ll dp[2][MAX_K + 1];

/* subroutines */

inline void setmax(ll &a, ll b) { if (a < b) a = b; }

/* main */

int main() {
  int n, k;
  ll p;
  scanf("%d%lld%d", &n, &p, &k);
  for (int i = 0; i < n; i++) scanf("%d%d", ts + i, bs + i);

  dp[0][0] = p;
  int cur = 0, nxt = 1;

  for (int i = 0; i < n; i++) {
    copy(dp[cur], dp[cur] + k + 1, dp[nxt]);
    for (int j = 0; j < k; j++)
      if (dp[cur][j] <= MAX_D) {
	ll dj = dp[cur][j] * ts[i] + bs[i];
	setmax(dp[nxt][j + 1], min(MAX_D + 1, dj));
      }
    swap(cur, nxt);
  }

  printf("%lld\n", (dp[cur][k] <= MAX_D) ? dp[cur][k] : -1LL);

  return 0;
}