import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Scanner;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
/**
 *
 * @author pttrung
 */
public class B {

    static Senator[] data;

    public static void main(String[] args) {
        Scanner in = new Scanner(new InputStreamReader(System.in));
        PrintWriter out = new PrintWriter(System.out);
        int n = in.nextInt();
        int k = in.nextInt();
        int A = in.nextInt();
        data = new Senator[n];
        for (int i = 0; i < n; i++) {
            data[i] = new Senator(in.nextInt(), in.nextInt());
        }
        out.println(cal(0, new int[n], A, k));

        out.close();
    }

    public static double cal(int index, int[] num, int A, int left) {
        if (index == data.length - 1) {
            int dif = (100 - data[index].loyal)/10;
            dif = left >= dif? dif:left;
            num[index] =  dif;
           
            double result = 0;
            for (int k = 0; k < (1 << num.length); k++) {
                double val = 1;
                double total = 0;
                for (int i = 0; i < num.length; i++) {
                    if (((1 << i) & k) != 0) {
                        val *= ((double)(data[i].loyal + 10*num[i])/100);
                    } else {
                        val *= ((double)(100 - (data[i].loyal + 10*num[i]))/100);
                        total += data[i].level;
                    }
                }
                if (countBit(k) > num.length / 2) {
                    result += val;
                } else {
                    result += val * ((double) A / (A + total));
                }
            }
//           // if(result >= 1){
//                for(int i : num){
//                    System.out.print(i + " ");
//                }
//                System.out.println("\n" + result);
//            //}
            return result;
        } else {
            double result = 0;
         ... What is the time complexity of the previous code?