source_code: import java.io.*;
  import java.util.*;
  
  public class TemnayaAssambleya implements Runnable {
  	public static void main(String[] args) {
  		new Thread(new TemnayaAssambleya()).run();
  	}
  
  	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  	StringTokenizer in;
  	PrintWriter out = new PrintWriter(System.out);
  
  	public String nextToken() throws IOException {
  		while (in == null || !in.hasMoreTokens()) {
  			in = new StringTokenizer(br.readLine());
  		}
  
  		return in.nextToken();
  	}
  
  	public int nextInt() throws IOException {
  		return Integer.parseInt(nextToken());
  	}
  
  	public double nextDouble() throws IOException {
  		return Double.parseDouble(nextToken());
  	}
  
  	int[] l;
  	int[] ln;
  	int[] pw;
  	int[] p;
  	int A;
  	double max = 0;
  
  	public void gen(int n, int k) {
  		if (n == 0) {
  			n = p.length;
  			p[0] = k;
  			for (int i = 0; i < n; i++) {
  				ln[i] = l[i] + p[i] * 10;
  				if (ln[i] > 100)
  					ln[i] = 100;
  			}
  
  			double ans = 0;
  			for (int mask = 0; mask < 1 << n; mask++) {
  				int z = 0;
  				double pos = 1;
  				int power = 0;
  				for (int i = 0; i < n; i++) {
  					if ((mask & (1 << i)) > 0) {
  						z++;
  						pos *= ln[i] * 1. / 100;
  					} else {
  						pos *= (100 - ln[i]) * 1. / 100;
  						power += pw[i];
  					}
  				}
  
  				if (z > n / 2) {
  					ans += pos;
  				} else {
  					ans += pos * A / (A + power);
  				}
  			}
  			
  			max = Math.max(ans, max);
  			return;
  		}
  
  		for (int i = 0; i <= Math.min(k, 10 - l[n] / 10); i++) {
  			p[n] = i;
  			gen(n - 1, k - i);
  		}
  	}
  
  	public void solve() throws IOException {
  		int n = nextInt();
  		int k = nextInt();
  		A = nextInt();
  
  		p = new int[n];
  		pw = new int[n];
  		l = new int[n];
  		ln = new int[n];
  
  		for (int i = 0; i < n; i++) {
  			pw[i] = nextInt();
  			l[i] = nextInt();
  		}
  
  		gen(n - 1, k);
  		out.println(max);
  	}
  
  	public void run() {
  		try {
  			solve();
  			out.close();
  		} catch (Exception e) {
  			e.printStackTrace();
  			System.exit(1);
  		}
  	}
  }
  