#include <bits/stdc++.h>
using namespace std;
int n, k;
double a, arr[10] = {}, arr2[10] = {}, calc[10] = {};
double check() {
  double ret = 0.0;
  for (int i = 0; i < (1 << n); ++i) {
    int votes = 0;
    double prob = 1, sum = 0.0;
    for (int j = 0; j < n; ++j) {
      if (i & (1 << j)) {
        votes++;
        prob *= (calc[j] / 100.0);
      } else {
        sum += arr[j];
        prob *= ((100.0 - calc[j]) / 100.0);
      }
    }
    if (votes > n / 2)
      ret += prob;
    else
      ret += (prob * (a / (sum + a)));
  }
  return ret;
}
double solve(int idx, int rem) {
  if (idx >= n) {
    if (!rem) return check();
    return 0;
  }
  double ret;
  ret = solve(idx + 1, rem);
  if (rem) {
    calc[idx] = min(100.0, calc[idx] + 10.0);
    ret = max(ret, solve(idx, rem - 1));
    calc[idx] = arr2[idx];
  }
  return ret;
}
int main() {
  ios::sync_with_stdio(0), ios_base::sync_with_stdio(0), cin.tie(0),
      cout.tie(0);
  ;
  cin >> n >> k >> a;
  for (int i = 0; i < n; ++i) {
    cin >> arr[i] >> arr2[i];
    calc[i] = arr2[i];
  }
  cout << fixed << setprecision(7) << solve(0, k) << "\n";
  return 0;
}
