#include <fstream>
#include <algorithm>

using namespace std;

ifstream cin("carnati.in");
ofstream cout("carnati.out");

struct ura
{
    int t,b;
}v[2001];
bool cmp(ura A, ura B) {
    return A.t < B.t;
}

int main()
{
    int n,c,i,maxi=-1000000000,s,aux,j;
    cin>>n>>c;
    for(i=1;i<=n;i++)
        cin>>v[i].t>>v[i].b;
    sort(v,v+n+1,cmp);
    v[0].t=v[1].t;
    for(i=1;i<=n;i++)
    {
        aux=v[i].b;s=0;
        for(j=1;j<=n;j++)
        {
            s=s-(v[j].t-v[j-1].t)*c;
            if(s<0)
                s=0;
            if(v[j].b>=aux)
                s=s+aux;
            if(s>maxi)
                maxi=s;
        }
    }
    cout<<maxi-c;
    return 0;
}