C++ :

#include <cstdio>
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
struct XX{
  int x, y;
}a[550]; 
bool cmp(const XX&x, const XX&y){
  if (x.x + x.y != y.x + y.y){
    return x.x + x.y < y.x + y.y;
  }
}
int dp[550][110];
int main(){
  int n, m;
  scanf("%d%d", &n, &m);
  for (int i = 0; i < n; ++i){
    scanf("%d%d", &a[i].x, &a[i].y);
  }
  sort(a, a + n, cmp);
  int ans = 0;
  for (int i = 0; i < n ; ++i){
    for (int k = 0; k <= m; ++k){
      dp[i][k] = 1;
    }
    for (int j = 0; j < i; ++j){
      if (a[i].x >= a[j].x && a[i].y >= a[j].y){
        int del = a[i].x - a[j].x + a[i].y - a[j].y - 1;
        for (int k = 0; k <= m - del; ++k){
          dp[i][k + del] = max(dp[i][k + del], dp[j][k] + del + 1);
          ans = max(ans, dp[i][k + del] + m - k - del);
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0; 
}