source_code: import java.io.InputStreamReader;
  import java.io.IOException;
  import java.util.InputMismatchException;
  import java.io.BufferedReader;
  import java.io.OutputStream;
  import java.io.PrintWriter;
  import java.io.Reader;
  import java.io.Writer;
  import java.io.InputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   * @author Niyaz Nigmatullin
   */
  public class Main {
  	public static void main(String[] args) {
  		InputStream inputStream = System.in;
  		OutputStream outputStream = System.out;
  		FastScanner in = new FastScanner(inputStream);
  		FastPrinter out = new FastPrinter(outputStream);
  		TaskB solver = new TaskB();
  		solver.solve(1, in, out);
  		out.close();
  	}
  }
  
  class TaskB {
  
      static int myLevel;
      static int[] level;
      static int[] loyalty;
  
      static double get(int n) {
          double ret = 0;
          for (int mask = 0; mask < 1 << n; mask++) {
              int k = Integer.bitCount(mask);
              double prob = 1.;
              int sum = 0;
              for (int i = 0; i < n; i++) {
                  if (((mask >> i) & 1) == 1) {
                      prob *= loyalty[i] * .1;
                  } else {
                      prob *= (10 - loyalty[i]) * .1;
                      sum += level[i];
                  }
              }
              if (k * 2 > n) {
                  ret += prob;
              } else {
                  ret += prob * myLevel / (myLevel + sum);
              }
          }
          return ret;
      }
  
      static double go(int x, int k, int n) {
          if (x == n) {
              return get(n);
          }
          double ret = 0;
          for (int i = 0; i <= k && loyalty[x] + i <= 10; i++) {
              loyalty[x] += i;
              ret = Math.max(go(x + 1, k - i, n), ret);
              loyalty[x] -= i;
          }
          return ret;
      }
  
      public void solve(int testNumber, FastScanner in, FastPrinter out) {
          int n = in.nextInt();
          int k = in.nextInt();
          myLevel = in.nextInt();
          level = new int[n];
          loyalty = new int[n];
          for (int i = 0; i < n; i++) {
              level[i] = in.nextInt();
              loyalty[i] = in.nextInt() / 10;
          }
          out.println(go(0, k, n));
      }
  }
  
  class FastScanner extends BufferedReader {
  
      boolean isEOF;
  
      public FastScanner(InputStream is) {
          super(new InputStreamReader(is));
      }
  
      public int read() {
          try {
              int ret = super.read();
              if (isEOF && ret < 0) {
                  throw new InputMismatchException();
              }
              isEOF = ret == -1;
              return ret;
          } catch (IOException e) {
              throw new InputMismatchException();
          }
      }
  
      static boolean isWhiteSpace(int c) {
          return c >= -1 && c <= 32;
      }
  
      public int nextInt() {
          int c = read();
          while (isWhiteSpace(c)) {
              c = read();
          }
          int sgn = 1;
          if (c == '-') {
              sgn = -1;
              c = read();
          }
          int ret = 0;
          while (!isWhiteSpace(c)) {
              if (c < '0' || c > '9') {
                  throw new NumberFormatException("digit expected " + (char) c
                          + " found");
              }
              ret = ret * 10 + c - '0';
              c = read();
          }
          return ret * sgn;
      }
  
      }
  
  class FastPrinter extends PrintWriter {
  
      public FastPrinter(OutputStream out) {
          super(out);
      }
  
      public FastPrinter(Writer out) {
          super(out);
      }
  
  
  }
  
  