import java.io.*;
import java.util.*;







public class B2 {
	String s = null;

	String[] ss = null;
	int[][] sn = null;
	int n = 0;
	double ans = 1;
	int A = 0;
	public void run() throws Exception{

		BufferedReader br = null;
		File file = new File("input.txt");
		if(file.exists()){
			br = new BufferedReader(new FileReader("input.txt"));
		}
		else{
			br = new BufferedReader(new InputStreamReader(System.in));
		}
	
		s = br.readLine();
		ss = s.split(" ");
		
		
		
		n = Integer.parseInt(ss[0]);
		int k = Integer.parseInt(ss[1]);
		A = Integer.parseInt(ss[2]);
		sn = new int[n][2];

		for(int i = 0; i < n; i++){
			s = br.readLine();
			ss = s.split(" ");
			sn[i][0] = Integer.parseInt(ss[0]);
			sn[i][1] = Integer.parseInt(ss[1]);
		}

		int num = 0;
		for(int i = 0; i < n; i++){
			num += (100 - sn[i][1]) / 10;
		}
		if(k >= num){
			System.out.println("1.0");
			return;
		}
		
		check(0, k, sn);
		
		ans = 1 - ans;
		System.out.println(ans);
		
	}
	
	void check(int i, int k, int[][] sn){
		if(i == n && k == 0){
			check2(sn);
			return;
		}
		else if(i == n && k > 0){
			return;
		}
		else if(k == 0){
			check(i+1, k, sn);
		}
		else{
			for(int j = 0; j <= k; j++){
				if(sn[i][1] + j * 10 <= 100){
					int[][] nsn = copy(sn);
					nsn[i][1] += j * 10;
					check(i+1, k - j, nsn);
				}
			}
		}
	}
	
	void check2(int[][] sn){
		List<Integer> target = new ArrayList<Integer>();
		int h = 0;
		for(int i = 0; i < sn.length; i++){
			if(sn[i][1] != 100){
				target.add(i);
			}
			else{
				h++;
			}
		}
		if(h  > n / 2){
			System.out.println("1.0");
			System.exit(0);	
		}
		int makemax = n - h;
		int makemin = (n+1)/2;
		double ma = 0;
		for(int i = makemax; i >= makemin; i--){
			Combination c = new Combination(makemax, i);
			Iterator<int[]> ite = c.iterator();
			
			while(ite.hasNext()){
				int[] ret = ite.next();
				Set<Integer> make = new HashSet<Integer>();
				for(int j = 0; j < ret.length; j++){
					if(ret[j] > 0){
						make.add(target.get(j));
					}
				}
				double makeK = 1;
				int B... Which one is the correct time complexity of the code snippet: constant, linear, quadratic, cubic, log(n), nlog(n) or NP-hard?