#include <bits/stdc++.h>
using namespace std;
int n, m, A;
int l[105], p[105], c[105];
double ans;
void cal() {
  double sum = 0;
  for (int i = 0; i < 1 << n; ++i) {
    double pr = 1.0;
    int s = 0, cnt = 0;
    for (int j = 0; j < n; ++j) {
      if (i & (1 << j))
        pr *= (p[j] + 10 * c[j]) / 100.0;
      else
        pr *= (100 - p[j] - c[j] * 10) / 100.0, ++cnt, s += l[j];
    }
    if (cnt * 2 < n)
      sum += pr;
    else
      sum += pr * A / (A + s);
  }
  ans = max(ans, sum);
}
void dfs(int d, int k) {
  if (d >= n) {
    cal();
    return;
  }
  for (int i = 0; i <= k; ++i) {
    c[d] = i;
    if (c[d] * 10 + p[d] > 100) break;
    dfs(d + 1, k - i);
  }
}
void work() {
  cin >> n >> m >> A;
  for (int i = 0; i < n; ++i) cin >> l[i] >> p[i];
  dfs(0, m);
  printf("%lf", ans);
}
int main() {
  work();
  return 0;
}
