#include <bits/stdc++.h>
using namespace std;
int n, k, A, b[11], B;
long double ll[11], c[11], m[11], x = 0, alfa = 0, maxi = 0;
bool bb[11];
void rec(int r, long double ver) {
  if (r == 0) {
    int t = 0;
    for (int i = 0; i <= n; i++)
      if (bb[i]) t++;
    if (t > n / 2)
      x += ver;
    else {
      B = 0;
      for (int i = 1; i <= n; i++)
        if (bb[i] == false) B += b[i];
      alfa = (long double)A / (long double)(A + B);
      x += ver * alfa;
    }
  } else {
    bb[r] = false;
    rec(r - 1, ver * (1 - m[r]));
    bb[r] = true;
    rec(r - 1, ver * m[r]);
  }
}
void rec2(int q, int l) {
  if (l == 0) {
    int t = 0;
    for (int i = 1; i <= n; i++)
      if (ll[i] + c[i] > 1.001) t = 1;
    if (t == 0) {
      for (int i = 1; i <= n; i++) {
        m[i] = ll[i] + c[i];
      }
      for (int i = 0; i <= n; i++) bb[i] = false;
      rec(n, 1);
      if (x > maxi) {
        maxi = x;
      }
      x = 0;
    }
  } else {
    for (int i = 0; i <= q; i++) {
      c[l] = (long double)i / 10;
      rec2(q - i, l - 1);
    }
  }
}
int main() {
  scanf("%d%d%d", &n, &k, &A);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &b[i]);
    cin >> ll[i];
    ll[i] /= 100;
  }
  rec2(k, n);
  cout << maxi;
  return 0;
}
