source_code: /**
   * Created by IntelliJ IDEA.
   * User: piyushd
   * Date: 3/26/11
   * Time: 10:53 PM
   * To change this template use File | Settings | File Templates.
   */
  
  
  public class TaskB {
  
      int[] levels;
      int[] loyalty;
  
      int n, k, A;
      double ans = Double.NEGATIVE_INFINITY;
  
      void rec(int ix, int sweets, int[] loyalty) {
          if (ix == n) {
  
              double nres = 0.0;
              for(int mask = 0; mask < (1<<n); mask++) {
  
                  double res = 1.0, totalStrength = 0;
                  for (int i = 0; i < n; i++) {
                      if ((mask & (1 << i)) > 0) {
                          res *= loyalty[i] / 100.0;
                      } else {
                          res *= 1.0 - (loyalty[i]/ 100.0);
                          totalStrength += levels[i];
                      }
                  }
  
                  int bitCount = Integer.bitCount(mask);
                  if(bitCount > n / 2) {
                      nres += res;
                  }
                  else {
                      nres += res * (A) / (A + totalStrength);
                  }
  
              }
              ans = Math.max(ans, nres);
              return;
          }
  
          for (int j = 0; j <= sweets; j++) {
              if (loyalty[ix] + 10 * j > 100) break;
              int[] nloyalty = loyalty.clone();
              nloyalty[ix] += 10 * j;
              rec(ix + 1, sweets - j, nloyalty);
          }
      }
  
      void run() {
          n = nextInt();
          k = nextInt();
          A = nextInt();
          levels = new int[n];
          loyalty = new int[n];
          for (int i = 0; i < n; i++) {
              levels[i] = nextInt();
              loyalty[i] = nextInt();
          }
  
          rec(0, k, loyalty);
          System.out.println(ans);
      }
  
      int nextInt() {
          try {
              int c = System.in.read();
              if (c == -1) return c;
              while (c != '-' && (c < '0' || '9' < c)) {
                  c = System.in.read();
                  if (c == -1) return c;
              }
              if (c == '-') return -nextInt();
              int res = 0;
              do {
                  res *= 10;
                  res += c - '0';
                  c = System.in.read();
              } while ('0' <= c && c <= '9');
              return res;
          } catch (Exception e) {
              return -1;
          }
      }
  
      long nextLong() {
          try {
              int c = System.in.read();
              if (c == -1) return -1;
              while (c != '-' && (c < '0' || '9' < c)) {
                  c = System.in.read();
                  if (c == -1) return -1;
              }
              if (c == '-') return -nextLong();
              long res = 0;
              do {
                  res *= 10;
                  res += c - '0';
                  c = System.in.read();
              } while ('0' <= c && c <= '9');
              return res;
          } catch (Exception e) {
              return -1;
          }
      }
  
      double nextDouble() {
          return Double.parseDouble(next());
      }
  
      String next() {
          try {
              StringBuilder res = new StringBuilder("");
              int c = System.in.read();
              while (Character.isWhitespace(c))
                  c = System.in.read();
              do {
                  res.append((char) c);
              } while (!Character.isWhitespace(c = System.in.read()));
              return res.toString();
          } catch (Exception e) {
              return null;
          }
      }
  
      String nextLine() {
          try {
              StringBuilder res = new StringBuilder("");
              int c = System.in.read();
              while (c == '\r' || c == '\n')
                  c = System.in.read();
              do {
                  res.append((char) c);
                  c = System.in.read();
              } while (c != '\r' && c != '\n');
              return res.toString();
          } catch (Exception e) {
              return null;
          }
      }
  
      public static void main(String[] args) {
          new TaskB().run();
      }
  }
  