#include <bits/stdc++.h>
using namespace std;
FILE *in = stdin;
int N, K, A, arr[10][2];
double ret;
int add[10];
void rec(int i, int rem) {
  if (rem == 0 || i == N) {
    double prob = 0;
    for (int m = 0; m < (1 << N); m++) {
      int B = 0, ok = 0;
      double tmp = 1.0;
      for (int q = 0; q < N; q++) {
        if (m & (1 << q)) {
          ok++;
          tmp *= (add[q] + arr[q][1]) / 100.0;
        } else {
          tmp *= 1 - (add[q] + arr[q][1]) / 100.0;
          B += arr[q][0];
        }
      }
      if (ok > N / 2) {
        prob += tmp;
      } else {
        tmp *= A / double(A + B);
        prob += tmp;
      }
    }
    ret = max(ret, prob);
    return;
  }
  rec(i + 1, rem);
  if (add[i] + arr[i][1] < 100) {
    add[i] += 10;
    rec(i, rem - 1);
    add[i] -= 10;
  }
  return;
}
int main() {
  fscanf(in, "%d%d%d", &N, &K, &A);
  for (int q = 0; q < N; q++) {
    fscanf(in, "%d%d", &arr[q][0], &arr[q][1]);
  }
  ret = 0;
  rec(0, K);
  printf("%lf\n", ret);
  return 0;
}
