#include <bits/stdc++.h>
using namespace std;
int n, k, A;
double mx_prob = 0;
int lvl[8], loyalty[8];
int candies[8];
void solve(int x, int k) {
  if (x == n) {
    for (int i = (0); i < ((n)); ++i) loyalty[i] += candies[i] * 10;
    double sum_p = 0;
    for (int bm = (0); bm < (((1 << n))); ++bm) {
      double p = 1.0;
      int sum_lvl = 0, cnt = 0;
      for (int i = (0); i < ((n)); ++i)
        if (bm & (1 << i))
          p *= loyalty[i] / 100.0;
        else {
          p *= (100 - loyalty[i]) / 100.0;
          sum_lvl += lvl[i];
          ++cnt;
        }
      if (cnt < (n / 2 + n % 2))
        sum_p += p;
      else
        sum_p += p * (A / (A + (double)sum_lvl));
    }
    for (int i = (0); i < ((n)); ++i) loyalty[i] -= candies[i] * 10;
    mx_prob = max(mx_prob, sum_p);
    return;
  }
  for (candies[x] = 0; candies[x] <= min(k, (10 - loyalty[x] / 10));
       ++candies[x])
    solve(x + 1, k - candies[x]);
}
int main() {
  scanf("%d%d%d", &n, &k, &A);
  for (int i = (0); i < ((n)); ++i) scanf("%d%d", &lvl[i], &loyalty[i]);
  solve(0, k);
  printf("%.6lf\n", mx_prob);
  return 0;
}
