source_code: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.FileNotFoundException;
  import java.util.StringTokenizer;
  import java.io.BufferedReader;
  import java.io.FileReader;
  import java.io.InputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   *
   * @author \/
   */
  public class Main {
      public static void main(String[] args) {
          InputStream inputStream = System.in;
          OutputStream outputStream = System.out;
          Scanner in = new Scanner(inputStream);
          PrintWriter out = new PrintWriter(outputStream);
          TaskC solver = new TaskC();
          solver.solve(1, in, out);
          out.close();
      }
  
      static class TaskC {
          public void solve(int testNumber, Scanner in, PrintWriter out) {
              int n = in.nextInt();
              int m = in.nextInt();
  
              TaskC.pair[] songs = new TaskC.pair[n];
              long sum = 0;
              for (int i = 0; i < n; i++) {
                  songs[i] = new TaskC.pair(in.nextInt(), in.nextInt());
                  sum += songs[i].a;
              }
              Arrays.sort(songs);
  
              int res = 0;
              int idx = n - 1;
              while (sum > m) {
                  if (idx < 0) {
                      break;
                  }
                  sum -= (songs[idx].a - songs[idx].b);
  
                  res++;
                  idx--;
              }
  
              if (sum > m) {
                  out.println(-1);
              } else {
                  out.println(res);
              }
          }
  
          static class pair implements Comparable<TaskC.pair> {
              int a;
              int b;
  
              pair(int a, int b) {
                  this.a = a;
                  this.b = b;
              }
  
              public int compareTo(TaskC.pair p) {
                  return (this.a - this.b) - (p.a - p.b);
              }
  
          }
  
      }
  
      static class Scanner {
          StringTokenizer st;
          BufferedReader br;
  
          public Scanner(InputStream s) {
              br = new BufferedReader(new InputStreamReader(s));
          }
  
          public Scanner(String s) {
              try {
                  br = new BufferedReader(new FileReader(s));
              } catch (FileNotFoundException e) {
                  e.printStackTrace();
              }
          }
  
          public String next() {
              while (st == null || !st.hasMoreTokens()) {
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              }
              return st.nextToken();
          }
  
          public int nextInt() {
              return Integer.parseInt(next());
          }
  
      }
  }
  
  