source_code: import java.util.Arrays;
  import java.util.PriorityQueue;
  import java.util.Scanner;
  
  
  
  public class Main {
      static double max = 0.0;
      public static void main(String[] args) {
          Scanner r = new Scanner(System.in);
          
          int n = r.nextInt();
          int k = r.nextInt();
          int A = r.nextInt();
          
          Person[] p = new Person[n];
          
          for(int i = 0; i < n; i++){         
              int l = r.nextInt();
              int prob = r.nextInt();
              
              p[i] = new Person(l, prob);
          }
          
          int[] add = new int[n];
          
          double res = dfs(0, k, p, add, n, A);
          
          System.out.println(res);
          
      }
      private static double dfs(int ptr, int k, Person[] p, int[] add, int n, int A) {
          if(k < 0)return 0;
          
          double res1 = 0;
          for(int m = 0; m < 1<<n; m++){
              double win = 1;
              int cnt = 0;
              for(int i = 0; i < n; i++){
                  if((m & (1 << i)) == 0){
                      win *= (100-(p[i].p+add[i]))*1.0/100;
                  }else{
                      win *= (add[i]+p[i].p)*1.0/100;                 
                      cnt++;
                  }
              }
  
              if(cnt > n/2){
                  res1 += win; 
              }else{
                  int B = 0;
                  for(int i = 0; i < n; i++){
                      if((m & (1 << i)) == 0){
                          B += p[i].l;
                      }
                  }
                  
                  win *= A*1.0/(A+B);
                                  
                  res1 += win; 
              }
          }
          
          double res2 = 0, res3 = 0;
          
          if(add[ptr]+p[ptr].p < 100){
              add[ptr] += 10;
              res2 = dfs(ptr, k-1, p, add, n, A);
              add[ptr] -= 10;
          }
          if(ptr+1 < n){
              res3 = dfs(ptr+1, k, p, add, n, A);
          }
          
          return Math.max(res1, Math.max(res2, res3));
      }
  }
  class Person{
      int l, p;
      public Person(int li, int pi){
          l = li;
          p = pi;
      }
      public String toString(){
          return String.format("[%d, %d]", l, p);
      }
  }