#include <bits/stdc++.h>
using namespace std;
pair<int, int> senator[16];
int N, K;
double A;
pair<int, int> sen[16];
double ans = 0;
void check() {
  int dp[8][8];
  memset(dp, 0, sizeof(dp));
  double res = 0;
  for (int i = 0; i < (1 << N); ++i) {
    int br = 0;
    double prob = 1.;
    double power = 0;
    for (int j = 0; j < N; ++j) {
      if (i & (1 << j)) {
        prob *= ((100 - sen[j].second) / 100.);
        ++br;
        power += sen[j].first;
      } else
        prob *= (sen[j].second / 100.);
    }
    if (br * 2 < N) {
      res += prob;
    } else {
      res += ((double)A / ((double)A + power) * prob);
    }
  }
  if (ans < res) ans = res;
}
void f(int idx, int left) {
  if (idx == N) {
    check();
    return;
  }
  sen[idx] = senator[idx];
  for (int i = 0; i <= left; ++i) {
    f(idx + 1, left - i);
    sen[idx].second = min(100, sen[idx].second + 10);
  }
}
int main() {
  cin >> N >> K >> A;
  for (int i = 0; i < N; ++i) cin >> senator[i].first >> senator[i].second;
  f(0, K);
  printf("%.6f\n", ans);
}
