#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-9;
const double pi = acos(-1.0);
const int N = 10;
int n, k, A;
int b[N], loy[N], newl[N];
int cnt[1 << N];
double ans;
double calc() {
  double ans = 0;
  for (int st = 0; st < (int)(1 << n); ++st) {
    double p = 1;
    int B = 0;
    for (int i = 0; i < (int)(n); ++i) {
      if ((st >> i) & 1) {
        p *= newl[i] / 100.0;
      } else {
        B += b[i];
        p *= 1.0 - newl[i] / 100.0;
      }
    }
    if (cnt[st] > n / 2)
      ans += p;
    else {
      ans += p * A / (A + B + 0.0);
    }
  }
  return ans;
}
void dfs(int id, int k) {
  if (id == n - 1) {
    newl[id] = min(loy[id] + k * 10, 100);
    ans = max(ans, calc());
    return;
  }
  for (int i = 0; i < (int)(k + 1); ++i) {
    newl[id] = min(loy[id] + i * 10, 100);
    dfs(id + 1, k - i);
  }
}
void solve() {
  for (int i = 0; i < (int)(n); ++i) {
    scanf("%d%d", b + i, loy + i);
  }
  ans = 0;
  dfs(0, k);
  printf("%.8lf\n", ans);
}
int main() {
  cnt[0] = 0;
  for (int i = 0; i < (int)(1 << N); ++i) cnt[i] = cnt[i >> 1] + (i & 1);
  while (~scanf("%d%d%d", &n, &k, &A)) {
    solve();
  }
  return 0;
}
