#include <bits/stdc++.h>
using namespace std;
const int maxint = 0x7FFFFFFF;
template <class T>
void show(T a, int n) {
  for (int i = 0; i < n; ++i) cout << a[i] << ' ';
  cout << endl;
}
template <class T>
void show(T a, int r, int l) {
  for (int i = 0; i < r; ++i) show(a[i], l);
  cout << endl;
}
int n, k, A, nn;
double ans, res;
int lev[32], loy[32];
void dfs2(double e, int cnt, int sum, int dep) {
  if (dep == n) {
    if (cnt > n / 2) {
      res += e;
    } else {
      res += e * A / (A + sum);
    }
    return;
  }
  dfs2(e * loy[dep] / 100., cnt + 1, sum, dep + 1);
  dfs2(e * (100 - loy[dep]) / 100., cnt, sum + lev[dep], dep + 1);
}
void calc() {
  res = 0;
  dfs2(1., 0, 0, 0);
  ans = max(ans, res);
}
void dfs(int k, int dep) {
  if (dep == n) {
    calc();
    return;
  }
  for (int i = 0; i <= k; ++i) {
    if (loy[dep] + i * 10 > 100) break;
    loy[dep] += i * 10;
    dfs(k - i, dep + 1);
    loy[dep] -= i * 10;
  }
}
int main() {
  scanf("%d%d%d", &n, &k, &A);
  nn = n / 2 + (n & 1);
  for (int i = 0; i < n; ++i) {
    scanf("%d%d", &lev[i], &loy[i]);
  }
  ans = 0;
  dfs(k, 0);
  printf("%.10f\n", ans);
  return 0;
}
