#include <fstream>
#include <iostream>

using namespace std;

ifstream f ("carnati.in");
ofstream g ("carnati.out");

const int NMAX = 2000;
const int TMAX = 15e2;
const int VMAX = 2e9;

struct pers{
  int timp;
  int pret;
}v[NMAX+1];
int sclav[TMAX+1];

int kadane(){
    int smax = -VMAX, sum = -VMAX;
    for(int j=1; j<=TMAX; j++){
        if(sum < 0)
            sum = 0;
        sum += sclav[j];
        smax = max(smax, sum);
    }
    return smax;
}

int main()
{
    int n, pret;
    f>>n>>pret;
    for(int i=1; i<=n; i++)
        f>>v[i].timp>>v[i].pret;
    
    int smax = -VMAX;
    for(int i=1; i<=n; i++){
        fill(sclav+1, sclav+1+TMAX, -1*pret);
        for(int j=1; j<=n; j++)
            if(v[i].pret <= v[j].pret)
                sclav[v[j].timp] += v[i].pret;
        smax = max(smax, kadane());
                
    }
    g << smax;
    
    return 0;
}