#include <cstdio>
#include <algorithm>
 
struct client{
int t, p;
} cl[2010];
 
bool cmp(client a, client b)
{
    return a.t<b.t;
}
 
int main()
{
    freopen("carnati.in","r",stdin);
    freopen("carnati.out","w",stdout);
 
    int n,c;
    scanf("%d%d",&n,&c);
    for(int i=1;i<=n;i++)
        scanf("%d%d",&cl[i].t,&cl[i].p);
    long long cost,max=0;
    std::sort(&cl[1],&cl[n+1],cmp);
    for(int j=1;j<=n;j++)
    {
        cost=cl[j].p;
        long long act=0;
        for(int i=1;i<=n;i++)
        {
            act=act-c*(cl[i].t-cl[i-1].t);
            if(cl[i].p>=cost)
            {
                if(act+cost>cost)
                {
                    act+=cost;
                }
                else
                    act=cost;
                if(act>max)
                {
                    max=act;
                }
            }
        }
    }
    printf("%lld",max-c);
    return 0;
}