#include <bits/stdc++.h>
using namespace std;
int N, K, A;
int B[9 + 5], H[9 + 5];
double Ans;
double Cal(int i, int Sum, int Supp) {
  if (i == N + 1) {
    if (Supp * 2 > N) return 1.0;
    return 1.0 * A / (A + Sum);
  }
  double ret = Cal(i + 1, Sum, Supp + 1) * H[i] / 100.0;
  ret += Cal(i + 1, Sum + B[i], Supp) * (1.0 - (H[i] / 100.0));
  return ret;
}
void dfs(int i, int r) {
  if (i == N + 1) {
    Ans = max(Ans, Cal(1, 0, 0));
    return;
  }
  int tmp = H[i];
  for (int j = tmp; j <= min(tmp + 10 * r, 100); j += 10)
    H[i] = j, dfs(i + 1, r - (j - tmp) / 10);
  H[i] = tmp;
}
int main() {
  scanf("%d%d%d", &N, &K, &A);
  for (int i = 1; i <= N; i++) scanf("%d%d", &B[i], &H[i]);
  dfs(1, K);
  printf("%.9f", Ans);
}
