#include <fstream>
using namespace std;
int v[2005],vp[2005],res,c=0,cp,mx=0;
int ssm(int sum)
{
    res=-2147483648;
    c=0;
    for(int i=0;i<=mx;i++)
    {
        c=c-cp;
        if(vp[i]>=sum)
            c=c+sum;
        if(res<c)
            res=c;
        if(c<0)
            c=0;
    }
    return res;
}
int main()
{
    ifstream cin("carnati.in");
    ofstream cout("carnati.out");
    int n,rmax=-2147483648,x,y;
    cin >> n >> cp;
    for(int i=0;i<n;i++)
    {
        cin >> x >> y;
        v[i]=y;
        vp[x]=y;
        if(x>mx)
            mx=x;
    }
    for(int i=0;i<n;i++)
    {
        if(ssm(v[i])>rmax)
            rmax=ssm(v[i]);
    }
    cout << rmax;
    return 0;
}