#include <bits/stdc++.h>
using namespace std;
int n, k, a;
int b[10], l[10], pp[10], sb;
bool sweet[10];
double tans, ans;
void Dfs(int pos, int chosen, int bSum, double p) {
  if (pos == n) {
    if (chosen > n / 2)
      tans += p;
    else {
      double tp = a;
      tp /= (sb - bSum + a);
      tans += p * tp;
    }
    return;
  }
  double np = (double)pp[pos] / 100;
  Dfs(pos + 1, chosen + 1, bSum + b[pos], p * np);
  Dfs(pos + 1, chosen, bSum, p * (1 - np));
}
void GiveSweet(int pos, int left) {
  if (pos == n) {
    tans = 0;
    Dfs(0, 0, 0, 1);
    ans = max(ans, tans);
    return;
  }
  pp[pos] = l[pos];
  GiveSweet(pos + 1, left);
  while (left && pp[pos] < 100) {
    pp[pos] += 10;
    GiveSweet(pos + 1, left - 1);
    left--;
  }
}
void GetAns() {
  ans = 0;
  GiveSweet(0, k);
  printf("%.12f\n", ans);
}
int main() {
  while (cin >> n >> k >> a) {
    sb = 0;
    for (int i = 0, _n(n); i < _n; ++i)
      scanf("%d%d", b + i, l + i), sb += b[i], pp[i] = l[i];
    GetAns();
  }
  return 0;
}
