#include <stdio.h>
#include <algorithm>

using namespace std;

#define MAX_N 2005
#define FIN "carnati.in"
#define FOUT "carnati.out"
#define INF 0x3f3f3f3f
#define f first
#define s second

int N, C, Res;
pair<int, int> A[MAX_N];

int main(void) 
{
    int i, j, k, p, t;

    freopen(FIN, "r", stdin);
    freopen(FOUT, "w", stdout);

    scanf("%d %d", &N, &C);
    for (i = 0; i < N; ++i)
        scanf("%d %d", &A[i].f, &A[i].s);

    sort(A, A+N);
    for (i = 0; i < N; ++i)
    {
         p = A[i].s; t = -INF;
         for (k = j = 0; j < N; ++j)
         {
             if (A[j].s < p) continue;
             t = max(t, A[j].f*C-k*p);
             Res = max(Res, k*p-A[j].f*C+t+p-C);
             k++;
         }
    }
    printf("%d\n", Res);

    return 0;
}