#include <bits/stdc++.h>
using namespace std;
int n, k, A;
int l[16], p[16];
double ans;
void gao(int idx, int rem) {
  if (idx == n) {
    double tp = 0;
    for (int m = 0; m < (1 << n); ++m) {
      int cnt = 0, sum = 0;
      double term = 1;
      for (int i = 0; i < n; ++i)
        if (m & (1 << i)) {
          term *= 1.0 * p[i] / 100;
          ++cnt;
        } else {
          sum += l[i];
          term *= 1.0 * (100 - p[i]) / 100;
        }
      if (2 * cnt <= n) term *= 1.0 * A / (A + sum);
      tp += term;
    }
    if (tp > ans) ans = tp;
    return;
  }
  for (int i = 0; i <= rem && p[idx] + i * 10 <= 100; ++i) {
    p[idx] += i * 10;
    gao(idx + 1, rem - i);
    p[idx] -= i * 10;
  }
}
int main() {
  while (scanf("%d%d%d", &n, &k, &A) != EOF) {
    for (int i = 0; i < n; ++i) scanf("%d%d", &l[i], &p[i]);
    ans = 0;
    gao(0, k);
    printf("%.8lf\n", ans);
  }
  return 0;
}
