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#include <bits/stdc++.h>
using namespace std;
const long long INF=1e18+1;
int main()
{ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    int n,c;
    cin>>n>>c;
    int MaxxKlocek=0;
    map<int,vector<int>>M;
    for(int i=1;i<=n;i++)
    {
        int a,b;
        cin>>a>>b;
        M[a].push_back(b);
        MaxxKlocek=max(MaxxKlocek,b);
    }
    vector<long long>W(MaxxKlocek+1,0);
    long long wyn=0;
    for(auto XD : M)
    {
        int wysokosc=XD.first;
        vector<int>U=XD.second;
        vector<long long>Prop;
        for(int u : U)
        {
            long long prop=max(W[u]+wysokosc,wyn+wysokosc-c);
            Prop.push_back(prop);
        }
        for(int i=0;i<U.size();i++)
        {
            W[U[i]]=max(W[U[i]],Prop[i]);
            wyn=max(wyn,W[U[i]]);
        }
    }
    cout<<wyn<<'\n';
    return 0;
}