#include <bits/stdc++.h>
using namespace std;
void prepare() {}
int a[10], n, k, A;
int b[10], l[10], nl[10];
double res;
double calc() {
  int _2n = 1 << n, k, sb, kk = 0;
  double r = 0;
  for (int mask = (0); mask < (_2n); mask++) {
    k = 0;
    sb = 0;
    double c = 1;
    bool ok = true;
    for (int i = (0); i < (((n))); i++) {
      if ((mask >> i) & 1) {
        k++;
        c *= nl[i] / 100.0;
      } else {
        if (nl[i] == 100) {
          ok = false;
          break;
        }
        c *= (100 - nl[i]) / 100.0;
        sb += b[i];
      }
    }
    if (ok) {
      kk++;
      if (2 * k > n)
        r += c;
      else
        r += c * (double)A / (A + sb);
    }
  }
  return r;
}
void rec(int cur, int ost) {
  if (cur + 1 == n) {
    a[cur] = ost;
    for (int i = (0); i < (((n))); i++) nl[i] = min(100, l[i] + 10 * a[i]);
    res = max(res, calc());
  } else {
    for (int i = 0; i <= ost; i++) {
      a[cur] = i;
      rec(cur + 1, ost - i);
    }
  }
}
void solve() {
  cin >> n >> k >> A;
  for (int i = (0); i < (((n))); i++) cin >> b[i] >> l[i];
  rec(0, k);
  printf("%.10lf\n", res);
}
int main() {
  prepare();
  solve();
  return 0;
}
