#include <cstdio>
#include <iostream>
#include <algorithm>
using namespace std;

#define NMAX 2005

struct client{
    int timp;
    int pret;
} v[ NMAX ];

bool cmp( client a, client b ){
    return a.timp < b.timp;
}

int main()
{
    freopen("carnati.in","r",stdin);
    freopen("carnati.out","w",stdout);

    int n, m, i, j, s, t, d, c, pret, program, maxi;
    maxi = -99999999;

    scanf("%d%d",&n,&c);
    for( i = 1; i <= n; ++i ) scanf("%d%d",&v[ i ].timp,&v[ i ].pret);

    sort( v + 1, v + 1 + n, cmp );

    for( i = 1; i <= n; ++i ){
        pret = v[ i ].pret;
        s = program = 0;
        for( j = 1; j <= n; ++j ){
            if( v[ j ].pret >= pret ){
                if( program == 0 ) program = v[ j ].timp - 1;
                s += pret - ( v[ j ].timp - program ) * c;
                program = v[ j ].timp;
                if( s < pret - c ) s = pret - c;
                if( s > maxi ) maxi = s;
            }
        }
    }

    printf("%d",maxi);

    return 0;

}