/**
 * Created at 19:38 on 2019-08-16
 */

import java.io.*;
import java.util.*;
import java.util.function.Predicate;

public class Main {

  static FastScanner sc = new FastScanner();
  static Output out = new Output(System.out);

  static final int[] dx = {0, 1, 0, -1};
  static final int[] dy = {-1, 0, 1, 0};

  static final long MOD = (long) (1e9 + 7);
  static final long INF = Long.MAX_VALUE / 2;

  public static class Solver {
    public Solver() {

      int N = sc.nextInt();

      long maxW = sc.nextLong();

      long[] V = new long[N];
      long[] W = new long[N];
      for (int i=0; i<N; i++) {
        V[i] = sc.nextLong();
        W[i] = sc.nextLong();
      }

      Baggage half[] = new Baggage[1<<N/2];

      for (Bit bit = new Bit(); !bit.get(N/2); bit.next()) {
        long w = 0, v = 0;
        for (int i = bit.nextSetBit(0); i>=0; i=bit.nextSetBit(i+1)) {
          w += W[i];
          v += V[i];
        }
        half[(int)bit.toLong()] = new Baggage(w, v);
      }

      Arrays.sort(half, (o1, o2) -> Compare.L(o1.weight, o2.weight));

      int validCount = 1;
      for (int i=1; i<(1<<N/2); i++) {
        if (half[i].value > half[validCount-1].value) {
          half[validCount++] = half[i];
        }
      }

      long ans = 0;

      for (Bit bit = new Bit(); !bit.get(N-N/2); bit.next()) {
        long w = 0, v = 0;

        for (int i = bit.nextSetBit(0); i>=0; i = bit.nextSetBit(i+1)) {
          w += W[N/2+i];
          v += V[N/2+i];
        }

        if (w > maxW) continue;

        final long wLimit = maxW - w;
        int index = BinarySearch.binarySearch((i) -> half[i].weight <= wLimit, 0, validCount, false);
        ans = Math.max(v + half[index].value, ans);
      }

      out.println(ans);

    }

    public static class Compare {

      public static int L(long l1, long l2) {
        if (l1 == l2) return 0;
        else if (l1 < l2) return -1;
        else return 1;
      }

    }

    public class Baggage {
      long weight, value;
      Baggage(long w, long v) {
        weight = w;
        value = v;
      }
    }


    public static class BinarySearch {

      /*
       * [l, r)の区間でchecker.test()を実行する
       * rightOK ... 右側OK左側NGならtrue, 右側NG左側OKならfalse
       */
      public static long binarySearchLong(Predicate<Long> checker, long l, long r, boolean rightOK) {
        long ng, ok;

        if (rightOK) {
          ok = r-1;
          ng = l;
          if (!checker.test(ok)) {
            return r;
          }
          if (checker.test(ng)) {
            return l;
          }
        } else {
          ok = l;
          ng = r-1;
          if (!checker.test(ok)) {
            return l-1;
          }
          if (checker.test(ng)) {
            return r-1;
          }
        }

        /* ok と ng のどちらが大きいかわからないことを考慮 */
        while (Math.abs(ok - ng) > 1) {
          long mid = (ok + ng) / 2;

          if (checker.test(mid)) ok = mid;
          else ng = mid;
        }

        return ok;
      }

      /*
       * [l, r)の区間でchecker.test()を実行する
       */
      public static int binarySearch(Predicate<Integer> checker, int l, int r, boolean rightOK) {

        int ng, ok;

        if (rightOK) {
          ok = r-1;
          ng = l;
          if (!checker.test(ok)) {
            return r;
          }
          if (checker.test(ng)) {
            return l;
          }
        } else {
          ok = l;
          ng = r-1;
          if (!checker.test(ok)) {
            return l-1;
          }
          if (checker.test(ng)) {
            return r-1;
          }
        }

        while (Math.abs(ok - ng) > 1) {
          int mid = (ok + ng) / 2;

          if (checker.test(mid)) ok = mid;
          else ng = mid;
        }

        return ok;
      }

      public static int lowerBound(long[] a, long v) {

        if (a[0] >= v) return 0;

        int l = 0, r = a.length;
        int mid = (r + l) / 2;

        while (r - l > 1) {

          if (a[mid] >= v) r = mid;
          else l = mid;

          mid = (r + l) / 2;
        }

        return r;

      }

      public static int upperBound(long[] a, long v) {

        if (a[0] > v) return 0;

        int l = 0, r = a.length;
        int mid = (r + l) / 2;

        while (r - l > 1) {

          if (a[mid] > v) r = mid;
          else l = mid;

          mid = (r + l) / 2;
        }

        return r;

      }

      public static int count(long[] a, long v) {
        int lower = lowerBound(a, v);
        int upper = upperBound(a, v);
        return upper - lower;
      }

    }


    public class Bit extends BitSet {

      public void set(long l) {
        clear();
        for (int i=0; i<64; i++) {
          if (((l>>i)&1) == 1) set(i, true);
        }
      }

      public long toLong() {
        long res = 0;
        for (int i = nextSetBit(0); i >= 0; i = nextSetBit(i+1)) {
          res += 1<<i;
        }
        return res;
      }

      public void next() {
        for (int i=0; ; i++) {
          if (get(i)) {
            set(i, false);
          } else {
            set(i, true);
            break;
          }
        }
      }

    }

  }

  public static void main(String[] args) {
    new Solver();
    out.flush();
  }

  static class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    private boolean hasNextByte() {
      if (ptr < buflen) {
        return true;
      } else {
        ptr = 0;
        try {
          buflen = in.read(buffer);
        } catch (IOException e) {
          e.printStackTrace();
        }
        if (buflen <= 0) {
          return false;
        }
      }
      return true;
    }

    private int readByte() {
      if (hasNextByte()) return buffer[ptr++];
      else return -1;
    }

    private static boolean isPrintableChar(int c) {
      return 33 <= c && c <= 126;
    }

    private void skipUnprintable() {
      while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
    }

    public boolean hasNext() {
      skipUnprintable();
      return hasNextByte();
    }

    public String next() {
      if (!hasNext()) throw new NoSuchElementException();
      StringBuilder sb = new StringBuilder();
      int b = readByte();
      while (isPrintableChar(b)) {
        sb.appendCodePoint(b);
        b = readByte();
      }
      return sb.toString();
    }

    public long nextLong() {
      if (!hasNext()) throw new NoSuchElementException();
      long n = 0;
      boolean minus = false;
      int b = readByte();
      if (b == '-') {
        minus = true;
        b = readByte();
      }
      if (b < '0' || '9' < b) {
        throw new NumberFormatException();
      }
      while (true) {
        if ('0' <= b && b <= '9') {
          n *= 10;
          n += b - '0';
        } else if (b == -1 || !isPrintableChar(b)) {
          return minus ? -n : n;
        } else {
          throw new NumberFormatException();
        }
        b = readByte();
      }
    }

    public int nextInt() {
      return (int) nextLong();
    }

    public int[] nextIntArray(int N, boolean oneBased) {
      if (oneBased) {
        int[] array = new int[N + 1];
        for (int i = 1; i <= N; i++) {
          array[i] = sc.nextInt();
        }
        return array;
      } else {
        int[] array = new int[N];
        for (int i = 0; i < N; i++) {
          array[i] = sc.nextInt();
        }
        return array;
      }
    }

    public long[] nextLongArray(int N, boolean oneBased) {
      if (oneBased) {
        long[] array = new long[N + 1];
        for (int i = 1; i <= N; i++) {
          array[i] = sc.nextLong();
        }
        return array;
      } else {
        long[] array = new long[N];
        for (int i = 0; i < N; i++) {
          array[i] = sc.nextLong();
        }
        return array;
      }
    }
  }

  static class Output extends PrintWriter {

    public Output(PrintStream ps) {
      super(ps);
    }

    public void print(int[] a, String separator) {
      for (int i = 0; i < a.length; i++) {
        if (i == 0) print(a[i]);
        else print(separator + a[i]);
      }
      println();
    }

    public void print(long[] a, String separator) {
      for (int i = 0; i < a.length; i++) {
        if (i == 0) print(a[i]);
        else print(separator + a[i]);
      }
      println();
    }

    public void print(String[] a, String separator) {
      for (int i = 0; i < a.length; i++) {
        if (i == 0) print(a[i]);
        else print(separator + a[i]);
      }
      println();
    }

    public void print(ArrayList a, String separator) {
      for (int i = 0; i < a.size(); i++) {
        if (i == 0) print(a.get(i));
        else print(separator + a.get(i));
      }
      println();
    }
  }

}



Measured memory_bytes: 95148