#include <bits/stdc++.h>
using namespace std;
double ans = 0;
int p[10];
int r[10];
int n, m, A;
void find(int a, int b) {
  if (a == n || b == 0) {
    double res = 0;
    for (int i = 0; i < (1 << n); i++) {
      double tmp = 1;
      int sum = 0, cnt = 0;
      for (int j = 0; j < n; j++)
        if ((i >> j) % 2 == 1) {
          cnt++;
          tmp *= double(p[j]) / 100;
        } else {
          sum += r[j];
          tmp *= double(100 - p[j]) / 100;
        }
      if (cnt * 2 > n)
        res += tmp;
      else {
        tmp *= double(A) / double(A + sum);
        res += tmp;
      }
    }
    ans = max(ans, res);
    return;
  }
  int tmp = p[a];
  for (int i = 0; i <= b && tmp + 10 * i <= 100; i++) {
    p[a] = tmp + 10 * i;
    find(a + 1, b - i);
  }
  p[a] = tmp;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin >> n >> m >> A;
  for (int i = 0; i < n; i++) cin >> r[i] >> p[i];
  find(0, m);
  cout << fixed << setprecision(10) << ans << endl;
  return 0;
}
