#include <bits/stdc++.h>
using namespace std;
template <typename T>
void Out(T res) {
  cout << res << endl;
  exit(0);
}
int L[10], B[10];
int n, A;
double solve() {
  double res = 0;
  for (int i = (0), _b(1 << n); i < _b; ++i) {
    int sb = 0, p = 0;
    double pr = 1;
    for (int j = (0), _b(n); j < _b; ++j) {
      if (!(i & (1 << j))) {
        sb += B[j];
        pr *= (double)(100 - L[j]) / 100.;
        p++;
      } else {
        pr *= (double)(L[j]) / 100.;
      }
    }
    if (p * 2 >= n) pr *= (double)A / (double)(A + sb);
    res += pr;
  }
  return res;
}
double Rec(int pos, int left) {
  if (pos == n) return solve();
  double res = Rec(pos + 1, left);
  int old = L[pos];
  for (int i = (1), _b(left + 1); i < _b; ++i) {
    L[pos] += 10;
    if (L[pos] > 100) break;
    res = max(res, Rec(pos + 1, left - i));
  }
  L[pos] = old;
  return res;
}
int main() {
  int k;
  cin >> n >> k >> A;
  for (int i = (0), _b(n); i < _b; ++i) cin >> B[i] >> L[i];
  double res = Rec(0, k);
  printf("%.10lf\n", res);
  return 0;
}
