#include <bits/stdc++.h>
using namespace std;
int n, k, A;
int b[9], l[9], ll[9];
double ret;
void init() {
  for (int i = 0; i < n; i++) scanf("%d%d", &b[i], &l[i]);
}
double calc() {
  for (int i = 0; i < n; i++) ll[i] = min(l[i], 100);
  double ret = 0;
  for (int i = 0; i < (1 << n); i++) {
    double p = 1.0;
    int cnt = 0, bb = 0;
    for (int j = 0; j < n; j++)
      if (i & (1 << j))
        p *= ll[j] / 100.0, cnt++;
      else
        p *= (100 - ll[j]) / 100.0, bb += b[j];
    if (p < 1e-8) continue;
    if (cnt > n / 2)
      ret += p;
    else
      ret += p * A / (A + bb);
  }
  return ret;
}
void dfs(int k, int p) {
  if (p == n) {
    ret = max(ret, calc());
    return;
  }
  for (int i = k; i >= 0; i--) {
    l[p] += i * 10;
    dfs(k - i, p + 1);
    l[p] -= i * 10;
  }
}
void solve() {
  ret = 0;
  dfs(k, 0);
  printf("%.10lf\n", ret);
}
int main() {
  while (scanf("%d%d%d", &n, &k, &A) != EOF) {
    init();
    solve();
  }
  return 0;
}
