#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int MAXN = 1000005;
int b[MAXN], l[MAXN];
int n, k, A;
int di[MAXN];
double ans;
double calc() {
  double res = 0.0;
  for (int i = (0); i < (1 << n); i++) {
    double r = 1.0;
    int cnt = 0;
    double force = 0.0;
    for (int j = (0); j < (n); j++) {
      if ((i >> j) & 1)
        cnt++, r *= min(1.0, l[j] / 100.0 + 0.1 * di[j]);
      else
        r *= 1.0 - min(1.0, l[j] / 100.0 + 0.1 * di[j]), force += b[j];
    }
    if (cnt > n / 2)
      res += r;
    else
      res += r * A / (force + A);
  }
  return res;
}
void dfs(int i, int left) {
  if (i == n - 1) {
    di[i] = left;
    ans = max(calc(), ans);
    return;
  }
  for (int j = (0); j < (left + 1); j++) {
    di[i] = j;
    dfs(i + 1, left - j);
  }
}
int main() {
  cin >> n >> k >> A;
  ans = 0;
  for (int i = (0); i < (n); i++) cin >> b[i] >> l[i];
  dfs(0, k);
  printf("%.8f", ans);
}
