#include <bits/stdc++.h>
using namespace std;
const int N = 8;
int bb[N], ll[N], cc[N];
int n, a;
double ans;
void update() {
  double sum = 0;
  for (int v = 0; v < 1 << n; v++) {
    double q = 1;
    int k = 0, b = 0;
    for (int i = 0; i < n; i++) {
      double p = min(ll[i] + cc[i] * 10, 100) / 100.0;
      if (v & 1 << i) {
        q *= p;
        k++;
      } else {
        q *= 1 - p;
        b += bb[i];
      }
    }
    if (k <= n / 2) q *= (double)a / (a + b);
    sum += q;
  }
  ans = max(ans, sum);
}
void solve(int i, int k) {
  if (i == n - 1) {
    cc[i] = k;
    update();
    return;
  }
  for (int h = 0; h <= k; h++) {
    cc[i] = h;
    solve(i + 1, k - h);
  }
}
int main() {
  int k;
  cin >> n >> k >> a;
  for (int i = 0; i < n; i++) cin >> bb[i] >> ll[i];
  solve(0, k);
  printf("%f\n", ans);
  return 0;
}
