#include<cstdio>
#include<algorithm>
using namespace std;
struct eu{long long t,val;};
eu v[2001];
int vec[2001],vc[2001],i,j,sum,n,m,k,cate,p1;
long long valmax,c;
int main ()
{
freopen("carnati.in","r",stdin);
freopen("carnati.out","w",stdout);
scanf("%d%lld",&n,&c);
for(i=1;i<=n;i++)
    {
    scanf("%lld%lld",&v[i].t,&v[i].val);
    vc[++cate]=v[i].val;
    }
sort(vc+1,vc+cate+1);
for(int qq=1;qq<=n;qq++)
    {
    int x=vc[qq];
    long long sum=0,p1=1;
    for(i=1;i<=n;i++)
        {
        if(v[i].val>=x)
            sum+=x;
        if(sum-(v[i].t-v[p1].t+1)*c>valmax)
            valmax=sum-(v[i].t-v[p1].t+1)*c;
        if(sum-(v[i].t-v[p1].t+1)*c<0)
            {
            sum=0;
            p1=i+1;
            }
        }
    }
printf("%lld",valmax);
return 0;
}