#include <bits/stdc++.h>
using namespace std;
pair<int, int> pii[10];
double ans = 0.0;
double cal(int n, int p, int vote, int A, int B) {
  if (n == p) {
    if (vote > n / 2)
      return 1.0;
    else
      return 1.0 * A / (A + B);
  }
  if (pii[p].second == 100) return cal(n, p + 1, vote + 1, A, B);
  double T = (1.0 * pii[p].second / 100);
  double sum = T * cal(n, p + 1, vote + 1, A, B);
  sum += (1.0 - T) * cal(n, p + 1, vote, A, B + pii[p].first);
  return sum;
}
void DFS(int n, int p, int k, int A) {
  if (p == n) {
    if (k) return;
    ans = max(ans, cal(n, 0, 0, A, 0));
    return;
  }
  for (int i = 0; i <= k; ++i) {
    if (pii[p].second + 10 * i > 100) {
      int T = 100 - pii[p].second;
      pii[p].second = 100;
      if (p == n - 1)
        DFS(n, p + 1, 0, A);
      else
        DFS(n, p + 1, k - T / 10, A);
      pii[p].second = 100 - T;
      return;
    } else {
      pii[p].second += 10 * i;
      DFS(n, p + 1, k - i, A);
      pii[p].second -= 10 * i;
    }
  }
}
int main() {
  int n, k, A;
  cin >> n >> k >> A;
  for (int i = 0; i < n; ++i) cin >> pii[i].first >> pii[i].second;
  DFS(n, 0, k, A);
  printf("%.10lf\n", ans);
  return 0;
}
