#include <bits/stdc++.h>
using namespace std;
double l[10], b[10];
vector<int> kk(10, -1);
vector<vector<int> > v;
int k, n;
void abc(int i, int sum) {
  if (i == 0 && sum == 0) {
    v.push_back(kk);
    return;
  }
  if (i == 0) {
    return;
  }
  for (int j = 0; j <= k; j++) {
    if (sum - j < 0) break;
    kk[i - 1] = j;
    abc(i - 1, sum - j);
  }
}
double A;
double ans = 0.0;
void solve(vector<int>& v) {
  double sum = 0.0;
  for (int i = 0; i < (1 << n); i++) {
    double cur = 1.0;
    int ed = 0;
    double B = 0.0;
    for (int j = 0; j < n; j++) {
      double z = (l[j] + v[j] * 10) * 0.01;
      z = min(z, 1.0);
      if (i & (1 << j)) {
        cur *= z;
        ed++;
      } else {
        cur *= 1.0 - z;
        B += b[j];
      }
    }
    if (ed <= n / 2) cur *= A / (A + B);
    sum += cur;
  }
  ans = max(ans, sum);
}
int main() {
  cin >> n >> k >> A;
  abc(n, k);
  for (int i = 0; i < n; i++) {
    cin >> b[i] >> l[i];
  }
  for (int i = 0; i < v.size(); i++) {
    solve(v[i]);
  }
  printf("%.9f", ans);
}
