source_code: //package round81;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.util.Scanner;
  
  public class B {
  	Scanner in;
  	PrintWriter out;
  	String INPUT = "";
  	
  	void solve()
  	{
  		int n = ni();
  		int k = ni();
  		int a = ni();
  		int[] lv = new int[n];
  		int[] lo = new int[n];
  		for(int i = 0;i < n;i++){
  			lv[i] = ni();
  			lo[i] = ni();
  		}
  		
  		out.printf("%.9f", rec(lv, lo, n, 0, k, a));
  	}
  	
  	double rec(int[] lv, int[] lo, int n, int pos, int k, int a)
  	{
  		if(pos == n){
  			int h = n/2+1;
  			double gp = 0;
  			for(int i = 0;i < 1<<n;i++){
  				if(Integer.bitCount(i) >= h){
  					double p = 1.0;
  					for(int j = 0;j < n;j++){
  						if(i<<31-j<0){
  							p *= (double)lo[j] / 100;
  						}else{
  							p *= (double)(100-lo[j]) / 100;
  						}
  					}
  					gp += p;
  				}else{
  					double p = 1.0;
  					int sl = 0;
  					for(int j = 0;j < n;j++){
  						if(i<<31-j<0){
  							p *= (double)lo[j] / 100;
  						}else{
  							p *= (double)(100-lo[j]) / 100;
  							sl += lv[j];
  						}
  					}
  					gp += p * a/(a+sl);
  				}
  			}
  			return gp;
  		}else{
  			int o = lo[pos];
  			double max = 0;
  			for(int i = 0;i <= k && lo[pos] <= 100;i++){
  				max = Math.max(max, rec(lv, lo, n, pos+1, k-i, a));
  				lo[pos]+=10;
  			}
  			lo[pos] = o;
  			return max;
  		}
  	}
  	
  	
  	
  	void run() throws Exception
  	{
  		in = oj ? new Scanner(System.in) : new Scanner(INPUT);
  		out = new PrintWriter(System.out);
  
  		long s = System.currentTimeMillis();
  		solve();
  		out.flush();
  		tr(System.currentTimeMillis()-s+"ms");
  	}
  	
  	public static void main(String[] args) throws Exception
  	{
  		new B().run();
  	}
  	
  	int ni() { return Integer.parseInt(in.next()); }
  	long nl() { return Long.parseLong(in.next()); }
  	double nd() { return Double.parseDouble(in.next()); }
  	boolean oj = System.getProperty("ONLINE_JUDGE") != null;
  	void tr(Object... o) { if(!oj)System.out.println(Arrays.deepToString(o)); }
  }
  