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

using namespace std;

#define CLEAR(x) memset(x, 0, sizeof(x))
#define maxim(a, b) ((a > b) ? a : b)
#define INF 2000000001
#define PII pair<int, int>
#define f first
#define s second
#define NMax 2048

int N, C, P[NMax], mx, bst;
PII v[NMax];

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

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

    sort(v+1, v+N+1);

    for (i = 1; i <= N; ++i)
    {
        CLEAR(P);
        for (j = i-1; j; --j)
            P[j] = P[j+1] + (v[i].s <= v[j].s) * v[i].s;
        P[i] = v[i].s;
        for (j = i+1; j <= N; ++j)
            P[j] = P[j-1] + (v[i].s <= v[j].s) * v[i].s;

        for (j = i, mx = -INF; j <= N; ++j)
            mx = maxim(mx, P[j] - v[j].f * C);

        P[i] = 0;
        for (j = 1; j <= i; ++j)
            bst = maxim(bst, mx + P[j] + v[j].f * C - C);
    }

    printf("%d\n", bst);

    return 0;    
}