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#include <bits/stdc++.h>
using namespace std;
long long nw[500000];
long long bs = 0;
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cerr.tie(0);
    long long n,c;
    cin>>n>>c;
    vector<pair<long long,int>> v(n);
    for(int i = 0;i<n;i++)
    {
        cin>>v[i].first>>v[i].second;
        v[i].second--;
    }
    int j =0;
    for(int i = 1;i<=500000;i++)
    {
        vector<pair<int,long long>> ch;
        while(v[j].first == i)
        {
            ch.push_back({v[j].second,max(v[j].first+bs-c,v[j].first+nw[v[j].second])});
            j++;
        }
        for(int q = 0;q<ch.size();q++)
        {
            bs = max(bs,ch[q].second);
            nw[ch[q].first] = max(nw[ch[q].first],ch[q].second);
        }
    }
    cout<<bs;
}