#include <stdio.h>
#include <string.h>

#define fin  "carnati.in"
#define fout "carnati.out"

const int Nmax = 2010;

struct inf
{
	int timp,cst;
};

int N,C;
inf v[Nmax];

long long best,a[Nmax];

inline long long max(long long a,long long b) { return (a>b)?(a):(b); }

void quicksort(int st,int dr)
{
	int m,i,j;
	inf aux;

	m = (st + dr) / 2;

	i = st; j = dr;

	do
	{
		while ( v[i].timp < v[m].timp ) ++i;
		while ( v[j].timp > v[m].timp ) --j;
		if ( i <= j )
		{
			aux = v[i];
			v[i] = v[j];
			v[j] = aux;
			++i;
			--j;
		}
	} while ( i < j );

	if ( i < dr ) quicksort(i,dr);
	if ( j > st ) quicksort(st,j);
}

void readsolve()
{
	int i,j,k,X;
    long long val;

	freopen(fin,"r",stdin);
	freopen(fout,"w",stdout);

	scanf("%d%d",&N,&C);

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

	quicksort(1,N);

	best = v[1].cst - C;
    v[0].timp = v[1].timp - 1;
    v[N+1].timp = -2;
    
    for ( i = 1; i <= N; ++i )
    {
        X = v[i].cst;
        
        memset(a,0,sizeof(a));
        
        for ( j = 1; j <= N; ++j )
        {
            val = 0;
            for ( k = j; v[k].timp == v[j].timp; ++k)
                if ( X <= v[k].cst ) 
                   val += X;
            --k;
            a[k] = max( a[j-1] - ( v[j].timp - v[j-1].timp ) * C + val,val-C);
            best = max(best,a[k]);
            j = k;
        }    
    }
    
	printf("%lld\n",best);
}

int main()
{
	readsolve();
	return 0;
}