#include <cstdio>
#include <algorithm>
#define NMAX 2015
#define INF 10000009

using namespace std;

int n,c,cat;
struct punct
{
    int t,p;
}v[NMAX];
int dp[NMAX];

bool cmp(punct r,punct p)
{
    if (r.t > p.t) return 0;
        else return 1;
}

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

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

    sort(v+1,v+n+1,cmp);
    int Max = -INF;
    for(int i = 1; i <= n; ++i)
    {
        int p = v[i].p;
        dp[0] = -INF;
        for(int j = 1; j <= n; ++j)
        {
            if (v[j].p  >= p) cat = p;
                else cat = 0;
            dp[j] = max(dp[j-1] - (v[j].t - v[j-1].t) * c + cat, cat - c);
            if (dp[j] > Max) Max = dp[j];
        }
    }
    printf("%d\n",Max);
    return 0;
}