#include <stdio.h>
#define max(a, b) ((a) > (b) ? (a) : (b))

int n, c;
int t[2001], p[2001];
long long int a[2001];
int i, j, k, g;
long long int prof = 0;

void Build(int x);
void Qsort(int st, int dr);

int main()
{
    freopen("carnati.in", "r", stdin);
    freopen("carnati.out", "w", stdout);
    scanf("%d %d", &n, &c);
    for ( i = 1; i <= n; i++ )
        scanf("%d %d", &t[i], &p[i]);
    Qsort(1, n);
    for ( i = 1; i <= n; i++ )
        Build(p[i]);
        
	printf("%d\n", prof);
    return 0; 
}

void Qsort(int st, int dr)
{
    int i = st-1;
    int j = dr+1;
    do
    {
        do { i++; } while ( t[i] < t[st] );
        do { j--; } while ( t[st] < t[j] );
        if ( i <= j )
        {
            int aux = t[i]; t[i] = t[j];t[j] = aux;    
            aux = p[i]; p[i] = p[j]; p[j] = aux;
        }
    } while ( i <= j );            
    if ( st < j ) Qsort(st, j);
    if ( i < dr ) Qsort(i, dr);
}

void Build(int x)
{
	for ( j = 1; j <= n; j++ )
		g = (x <= p[j] ? x : 0),
		a[j] = max(a[j-1]-(t[j] - t[j-1]) * c + g, g-c),
		prof = prof < a[j] ? a[j] : prof;
}