#include <bits/stdc++.h>
using namespace std;
vector<pair<int, int> > s;
int n, a;
double calc(int v, int p, int A, int B) {
  if (p >= n) {
    if (v > n / 2) return 1.0;
    return (double)A / (A + B);
  }
  int x;
  double ans = 0;
  x = min(s[p].second, 100);
  ans += x / 100.0 * calc(v + 1, p + 1, A, B);
  ans += (100 - x) / 100.0 * calc(v, p + 1, A, B + s[p].first);
  return ans;
}
double rec(int k, int p) {
  if (k < 1) return calc(0, 0, a, 0);
  int i, j;
  double ans = 0;
  for (i = p; i < n; i++) {
    for (j = 0; j <= k; j++) {
      s[i].second += 10 * j;
      ans = max(rec(k - j, i + 1), ans);
      s[i].second -= 10 * j;
    }
  }
  return ans;
}
int main() {
  int k;
  int i, x, y;
  cin >> n >> k >> a;
  for (i = 0; i < n; i++) {
    cin >> x >> y;
    s.push_back(pair<int, int>(x, y));
  }
  printf("%.10lf\n", rec(k, 0));
  return 0;
}
