#include <fstream>
using namespace std;
ifstream cin("carnati.in");
ofstream cout("carnati.out");
int v[2005],h[2005],n,cp;
int ssm(int sum)
{
    int res=-cp,c=0,timp=1;
    for (int i=0; i<n; i++)
    {
        c=c-timp*cp;
        timp=v[i+1]-v[i];
        if(h[i]>=sum)
            c = c + sum;
        if(c>res)
            res=c;
        if(c<0)
        {
            c=0;
            timp=1;
        }
    }
    return res;
}
int main()
{
    int x,y,output=0,xy;
    cin >> n >> cp;
    for(int i=0;i<n;i++)
    {
        cin >> x >> y;
        v[i]=x;
        h[i]=y;
    }
    for(int i=0;i<n;i++)
    {
        xy=ssm(h[i]);
        if(xy>output)
            output=xy;
    }
    cout << output;
    return 0;
}