#include <bits/stdc++.h>
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int N,C,sol,D[1<<11];
pair <int,int> v[1<<11];
int main()
{
    f>>N>>C;
    for(int i=1;i<=N;i++)
        f>>v[i].first>>v[i].second;
    sort(v+1,v+N+1);
    v[0].first=v[1].first-1;
    for(int j=1;j<=N;j++)
        for(int i=1;i<=N;i++)
        {
            D[i]=0;
            if(v[j].second<=v[i].second) D[i]=v[j].second;
            D[i]=max(D[i]+D[i-1]-(v[i].first-v[i-1].first)*C,D[i]-C);
            sol=max(sol,D[i]);
        }
    g<<sol;
    return 0;
}