#include <bits/stdc++.h>
using namespace std;
int n, mid, A, a[11], b[11], k;
double ans;
inline void ev(int dep, int cnt, double pro, int deny, double &sum) {
  if (!pro) return;
  if (dep == n) {
    if (cnt >= mid)
      sum += pro;
    else
      sum += pro * A / (A + deny);
    return;
  }
  ev(dep + 1, cnt + 1, pro * a[dep + 1] / 10.0, deny, sum);
  ev(dep + 1, cnt, pro - pro * a[dep + 1] / 10.0, deny + b[dep + 1], sum);
}
inline void dfs(int dep, int swe) {
  if (!swe) {
    register double sum = 0;
    ev(0, 0, 1, 0, sum);
    ans = ans > sum ? ans : sum;
  }
  if (dep == n) return;
  for (register int i = 0; i <= swe; ++i)
    if (a[dep + 1] + i <= 10)
      a[dep + 1] += i, dfs(dep + 1, swe - i), a[dep + 1] -= i;
}
signed main() {
  scanf("%d%d%d", &n, &k, &A);
  mid = n / 2 + 1;
  for (register int i = 1; i <= n; ++i) scanf("%d%d", &b[i], &a[i]), a[i] /= 10;
  for (register int i = 0; i <= k; ++i) dfs(0, i);
  return !printf("%lf", ans);
  ;
}
