source_code: import java.io.BufferedReader;
  import java.io.FileReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.util.Locale;
  import java.util.StringTokenizer;
  import java.util.TreeMap;
  import java.util.Map.Entry;
  
  public class Solution implements Runnable {
  
  	public static void main(String[] args) {
  		(new Thread(new Solution())).start();
  	}
  	
  	BufferedReader in;
  	PrintWriter out;
  	StringTokenizer st;
  	
  	String nextToken() throws IOException {
  		while (st == null || !st.hasMoreTokens()) {
  			String r = in.readLine();
  			if (r == null) return null;
  			st = new StringTokenizer(r);
  		}
  		return st.nextToken();
  	}
  
  	int nextInt() throws IOException {
  		return Integer.parseInt(nextToken());
  	}
  
  	long nextLong() throws IOException {
  		return Long.parseLong(nextToken());
  	}
  
  	double nextDouble() throws IOException {
  		return Double.parseDouble(nextToken());
  	}
  	
  	int n;
  	int[] b, l;
  	double ans;
  	int[] sw;
  	int a;
  	
  	void sol() {
  		for (int i = 0; i < n; i++) l[i] += sw[i] * 10;
  		double yes = 0;
  		for (int q = 0; q < (1 << n); q++) {
  			double p = 1;
  			int bb = 0;
  			int cnt = 0;
  			for (int i = 0; i < n; i++) {
  				if ((q & (1 << i)) == 0) {
  					p *= (1.0 - (double)l[i] / 100);
  					bb += b[i];
  				} else {
  					p *= 1.0 * (double)l[i] / 100;
  					cnt++;
  				}
  			}
  			if (cnt > n / 2) {
  				yes += p;
  			} else {
  				yes += p * (double)a / (double)(a + bb); 
  			}
  		}
  		if (ans < yes) ans = yes;
  		for (int i = 0; i < n; i++) l[i] -= sw[i] * 10;
  	}
  	
  	void rek(int i, int k) {
  		if (i == n) sol(); else {
  			for (int q = 0; q <= k && l[i] + q * 10 <= 100; q++) {
  				sw[i] = q;
  				rek(i + 1, k - q);
  			}
  		}
  	}
  	
  	void solve() throws Exception {
  		n = nextInt();
  		int k = nextInt();
  		a = nextInt();
  		b = new int[n];
  		l = new int[n];
  		sw = new int[n];
  		for (int i = 0; i < n; i++) {
  			b[i] = nextInt();
  			l[i] = nextInt();
  		}
  		rek(0, k);
  		out.printf("%.10f", ans);
  	}
  
  	public void run() {
  		Locale.setDefault(Locale.UK);
  		try {
  			in = new BufferedReader(new InputStreamReader(System.in));
  			out = new PrintWriter(System.out);
  //			in = new BufferedReader(new FileReader("knights.in"));
  //			out = new PrintWriter("knights.out");
  			solve();
  		} catch (Exception e) {
  			e.printStackTrace();
  			System.exit(1);
  		} finally {
  			out.flush();
  		}
  	}
  
  }