#include <bits/stdc++.h>
using namespace std;
ifstream fin("carnati.in");
ofstream fout("carnati.out");
int n,sol,C,D[1<<11];
struct pr{
int a,b;
}v[1<<11];
bool comp(pr a,pr b)
{
    return a.a<b.a;
}
void rezolvare()
{
    v[0].a = v[1].a - 1;
    for (int j=1;j<=n;j++)
         for (int i=1;i<=n;i++)
    {
        D[i] = 0;
        if (v[j].b<=v[i].b) D[i] = v[j].b;
        D[i] = max(D[i]+D[i-1]-(v[i].a-v[i-1].a)*C,D[i]-C);
        sol = max (sol,D[i]);
    }
}
int main()
{
    fin >> n >> C;
    for (int i=1;i<=n;i++)
        fin >> v[i].a >> v[i].b;
    sort(v+1,v+n+1,comp);
    rezolvare();
    fout << sol << '\n';
    return 0;
}