#include <bits/stdc++.h>
using namespace std;
int N, K, A;
int b[10], L[10];
int tL[10];
double ans;
void Calc() {
  double sum = 0.0;
  for (int i = 0; i < (1 << N); ++i) {
    double temp = 1.0;
    double kill = 1.0;
    int ct = 0;
    int B = 0;
    for (int j = 0; j < N; ++j) {
      if (i & (1 << j)) {
        temp = temp * tL[j] * 0.01;
        ct++;
      } else {
        temp = temp * (100 - tL[j]) * 0.01;
        B += b[j];
      }
    }
    if (ct * 2 > N) {
      sum += temp;
    } else {
      sum += temp * (double)A / (double)(A + B);
    }
  }
  if (sum > ans) ans = sum;
}
void DFS(int index, int left) {
  if (index == N - 1) {
    int t = L[index] + left * 10;
    tL[index] = t > 100 ? 100 : t;
    Calc();
  } else {
    for (int i = 0; i <= left; ++i) {
      int t = L[index] + i * 10;
      tL[index] = t > 100 ? 100 : t;
      DFS(index + 1, left - i);
    }
  }
}
int main() {
  while (cin >> N >> K >> A) {
    for (int i = 0; i < N; ++i) {
      cin >> b[i] >> L[i];
    }
    ans = 0.0;
    DFS(0, K);
    printf("%.6lf\n", ans);
  }
  return 0;
}
