source_code: import java.io.InputStream;
  import java.io.OutputStream;
  import java.io.IOException;
  import java.util.ArrayList;
  import java.io.InputStream;
  import java.io.BufferedWriter;
  import java.io.Writer;
  import java.io.OutputStreamWriter;
  import java.io.IOException;
  import java.util.InputMismatchException;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.io.OutputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   *
   * @author Alex
   */
  public class Main {
      public static void main(String[] args) {
          InputStream inputStream = System.in;
          OutputStream outputStream = System.out;
          InputReader in = new InputReader(inputStream);
          OutputWriter out = new OutputWriter(outputStream);
          TaskB solver = new TaskB();
          solver.solve(1, in, out);
          out.close();
      }
      static class TaskB {
          double special(int[] loyalties, int[] levels, int playerlevelsum) {
              int poss = 1 << loyalties.length;
              double res = 0;
              for(int pos = 0; pos < poss; pos++) {
                  double occurs = 1;
                  int happy = 0;
                  int badlevelssum = 0;
                  for(int i = 0; i < loyalties.length; i++) {
                      if(((pos >> i) & 1) == 1) { //happy senator
                          happy++;
                          occurs *= (double) loyalties[i] / 100;
                      } else { //unhappy senator
                          badlevelssum += levels[i];
                          occurs *= (double) (100 - loyalties[i]) / 100;
                      }
                  }
                  double winprob = (happy <= levels.length / 2) ? (double) playerlevelsum / (playerlevelsum + badlevelssum) : 1;
                  res += occurs * winprob;
              }
              return res;
          }
  
          public void solve(int testNumber, InputReader in, OutputWriter out) {
              int senators = in.readInt(); // n, [1, 8]
              int sweets = in.readInt(); // k, [1, 8]
              int playerlevelsum = in.readInt(); // A, [1, 9999]
              int[] levels = new int[senators]; // [1, 9999]
              int[] loyalties = new int[senators]; // [0, 100] divisible by 10
              IOUtils.readIntArrays(in, levels, loyalties);
              ArrayList<ArrayList<Integer>> possibilities = new ArrayList<>(Arrays.asList(new ArrayList<>()));
              for(int senator = 0; senator < senators; senator++) {
                  ArrayList<ArrayList<Integer>> newpossibilities = new ArrayList<>();
                  for(ArrayList<Integer> al : possibilities) {
                      int sumsofar = 0;
                      for(int val : al) sumsofar += val;
                      int minadd = senator == senators - 1 ? sweets - sumsofar : 0;
                      for(int moar = minadd; moar <= sweets - sumsofar; moar++) {
                          ArrayList<Integer> copy = new ArrayList<>(al);
                          copy.add(moar);
                          newpossibilities.add(copy);
                      }
                  }
                  possibilities = newpossibilities;
              }
              double res = 0;
              for(ArrayList<Integer> al : possibilities) {
                  int[] newloyalties = new int[senators];
                  for(int i = 0; i < senators; i++) newloyalties[i] = Math.min(100, loyalties[i] + 10 * al.get(i));
                  double special = special(newloyalties, levels, playerlevelsum);
                  double tot = special;
                  res = Math.max(res, tot);
              }
              out.printLine(res);
          }
  
      }
  
      static class InputReader {
          private InputStream stream;
          private byte[] buf = new byte[1024];
          private int curChar;
          private int numChars;
          private SpaceCharFilter filter;
  
          public InputReader(InputStream stream) {
              this.stream = stream;
          }
  
          public int read() {
              if(numChars == -1)
                  throw new InputMismatchException();
              if(curChar >= numChars) {
                  curChar = 0;
                  try {
                      numChars = stream.read(buf);
                  } catch(IOException e) {
                      throw new InputMismatchException();
                  }
                  if(numChars <= 0)
                      return -1;
              }
              return buf[curChar++];
          }
  
          public int readInt() {
              int c = read();
              while(isSpaceChar(c))
                  c = read();
              int sgn = 1;
              if(c == '-') {
                  sgn = -1;
                  c = read();
              }
              int res = 0;
              do {
                  if(c < '0' || c > '9')
                      throw new InputMismatchException();
                  res *= 10;
                  res += c - '0';
                  c = read();
              } while(!isSpaceChar(c));
              return res * sgn;
          }
  
          public boolean isSpaceChar(int c) {
              if(filter != null)
                  return filter.isSpaceChar(c);
              return isWhitespace(c);
          }
  
          public static boolean isWhitespace(int c) {
              return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
          }
  
          public interface SpaceCharFilter {
              public boolean isSpaceChar(int ch);
  
          }
  
      }
  
      static class OutputWriter {
          private final PrintWriter writer;
  
          public OutputWriter(OutputStream outputStream) {
              writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
          }
  
          public OutputWriter(Writer writer) {
              this.writer = new PrintWriter(writer);
          }
  
          public void print(Object... objects) {
              for(int i = 0; i < objects.length; i++) {
                  if(i != 0)
                      writer.print(' ');
                  writer.print(objects[i]);
              }
          }
  
          public void printLine(Object... objects) {
              print(objects);
              writer.println();
          }
  
          public void close() {
              writer.close();
          }
  
      }
  
      static class IOUtils {
          public static void readIntArrays(InputReader in, int[]... arrays) {
              for(int i = 0; i < arrays[0].length; i++) {
                  for(int j = 0; j < arrays.length; j++)
                      arrays[j][i] = in.readInt();
              }
          }
  
      }
  }
  
  