source_code: import java.awt.geom.*;
  import java.io.*;
  import java.math.*;
  import java.util.*;
  import java.util.regex.*;
  import static java.lang.Math.*;
  public class B {
  
      public static void swap(int[] array, int a, int b) { // {{{
          int t = array[a];
          array[a] = array[b];
          array[b] = t;
      } // }}}
  
      int n, k, A;
      int[] b;
      int[] l;
  
      double ans;
  
      double cal() {
          double total = 0;
          for (int mask=0;mask<(1<<n);mask++) {
              int bit = Integer.bitCount(mask);
              double p = 1;
              for (int i=0;i<n;i++) {
                  if ((mask&(1<<i))!=0)
                      p *= l[i]/10.0;
                  else 
                      p *= 1.0-l[i]/10.0;
              }
  
              if (bit*2>n) 
                  total += p;
              else {
                  int B = 0;
                  for (int i=0;i<n;i++) {
                      if ((mask&(1<<i))==0) {
                          B += b[i];
                      }
                  }
                  total += p*(A/(double)(A+B));
              }
          }
          return total;
      }
  
      void rec(int d, int remain) {
          ans = max(ans, cal());
          if (remain==0) return;
          for (int i=d;i<n;i++) {
              if (l[i]==10) continue;
              l[i]++;
              rec(i, remain-1);
              l[i]--;
          }
      }
  
      public B() throws Exception {
          n = in.nextInt();
          k = in.nextInt();
          A = in.nextInt();
  
          b = new int[n];
          l = new int[n];
          for (int i=0;i<n;i++) {
              b[i] = in.nextInt();
              l[i] = in.nextInt()/10;
          }
  
          ans = 0;
          rec(0, k);
  
          System.out.print(ans);
      }
  
      Scanner in = new Scanner(System.in);
      StringBuilder buf = new StringBuilder();
      public static void main(String[] args) throws Exception { // {{{
          new B();
      } // }}}
      public static void debug(Object... arr) { // {{{
          System.err.println(Arrays.deepToString(arr));
      } // }}}
  }
  