#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("carnati.in");
ofstream fout("carnati.out");
long int dp[2001],i,j,n,c,solutie,pr,val,Max;
struct structura
{
    int t;
    int p;
}v[2001];
int cmp(structura o, structura p1)
{
    return o.t<p1.t;
}
int main()
{
    fin>>n>>c;
    for(i=1;i<=n;i++)
        fin>>v[i].t>>v[i].p;
    sort(v+1,v+n+1,cmp);
    v[0].t=-1;
    for(i=1;i<=n;i++)
    {
        val=v[i].p;
        for(j=1;j<=n;j++)
        {
            if(val<=v[j].p)
                pr=val;
            else
                pr=0;
            dp[j]=max(dp[j-1]+pr-c*(v[j].t-v[j-1].t),pr-c);
            if(dp[j]>Max)
                Max=dp[j];
        }
    }
    fout<<Max;
    return 0;
}