source_code: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.PrintWriter;
  import java.util.StringTokenizer;
  import java.io.IOException;
  import java.io.BufferedReader;
  import java.io.InputStreamReader;
  import java.io.InputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   */
  public class Main {
      public static void main(String[] args) {
          InputStream inputStream = System.in;
          OutputStream outputStream = System.out;
          InputReader in = new InputReader(inputStream);
          PrintWriter out = new PrintWriter(outputStream);
          TaskB solver = new TaskB();
          solver.solve(1, in, out);
          out.close();
      }
  
      static class TaskB {
          double ans = 0;
          int[] candy;
          int[] b;
          int[] l;
          int[] curLoyal;
          int n;
          int k;
          int A;
  
          public void solve(int testNumber, InputReader in, PrintWriter out) {
              n = in.nextInt();
              k = in.nextInt();
              A = in.nextInt();
              b = new int[n];
              l = new int[n];
              for (int i = 0; i < n; i++) {
                  b[i] = in.nextInt();
                  l[i] = in.nextInt();
              }
              candy = new int[n];
              curLoyal = new int[n];
              comb(0, k);
              out.println(ans);
          }
  
          void comb(int ind, int unusedCandy) {
              if (ind == n) {
                  for (int i = 0; i < n; i++) {
                      curLoyal[i] = Math.min(candy[i] * 10 + l[i], 100);
                  }
                  calc();
              } else {
                  for (int i = 0; i <= unusedCandy; i++) {
                      candy[ind] = i;
                      comb(ind + 1, unusedCandy - i);
                  }
              }
          }
  
          void calc() {
              double res = 0;
              int allBits = (1 << n) - 1;
              for (int vote = 0; vote <= allBits; vote++) {
                  double curProb = 1;
                  int sumPower = A;
                  int enemyCnt = 0;
                  for (int voteInd = 0; voteInd < n; voteInd++) {
                      if ((vote & (1 << voteInd)) == 0) {
                          curProb *= 100 - curLoyal[voteInd];
                          sumPower += b[voteInd];
                          enemyCnt++;
                      } else {
                          curProb *= curLoyal[voteInd];
                      }
                      curProb /= 100;
                  }
                  if (2 * enemyCnt >= n) {
                      curProb *= A;
                      curProb /= sumPower;
                  }
                  res += curProb;
              }
              ans = Math.max(ans, res);
          }
  
      }
  
      static class InputReader {
          private BufferedReader reader;
          private StringTokenizer stt;
  
          public InputReader(InputStream stream) {
              reader = new BufferedReader(new InputStreamReader(stream));
          }
  
          public String nextLine() {
              try {
                  return reader.readLine();
              } catch (IOException e) {
                  return null;
              }
          }
  
          public String next() {
              while (stt == null || !stt.hasMoreTokens()) {
                  stt = new StringTokenizer(nextLine());
              }
              return stt.nextToken();
          }
  
          public int nextInt() {
              return Integer.parseInt(next());
          }
  
      }
  }
  
  