#include <bits/stdc++.h>
using namespace std;
const int N = 8;
int lev[N], loy[N], add[N];
int n, k, A;
double ans;
void solve() {
  double p = 0;
  for (int mask = 0; mask < (1 << n); mask++) {
    double a = 1.0;
    int sum = 0, num = 0;
    for (int i = 0; i < n; i++) {
      double x = 1.0 * (loy[i] + add[i] * 10) / 100;
      if (mask >> i & 1)
        a *= x;
      else {
        a *= 1 - x;
        num++;
        sum += lev[i];
      }
    }
    if (num * 2 < n)
      p += a;
    else
      p += a * A / (A + sum);
  }
  if (p > ans) ans = p;
}
void rec(int p, int k) {
  if (p >= n) {
    solve();
    return;
  }
  for (int i = 0; i <= k; i++) {
    add[p] = i;
    if (loy[p] + i * 10 > 100) return;
    rec(p + 1, k - i);
  }
}
int main() {
  scanf("%d %d %d", &n, &k, &A);
  for (int i = 0; i < n; i++) {
    scanf("%d %d", &lev[i], &loy[i]);
  }
  ans = 0;
  rec(0, k);
  printf("%.10lf\n", ans);
  return 0;
}
