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#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    ll n, c;
    cin>>n>>c;
    vector<pair<ll, set<ll>>>v;
    unordered_map<ll, ll>m;
    for(int i=0; i<n; i++)
    {
        ll cur, curr;
        cin>>cur>>curr;
        m[curr]=0;
        if(i==0||v.back().first!=cur)
        {
            v.push_back({cur, (set<ll>){curr}});
        }
        else
            v.back().second.insert(curr);
    }
    v.push_back({0, (set<ll>){0}});
    reverse(v.begin(), v.end());
    int siz=v.size();
    vector<ll>dp(siz, 0);
    for(int i=1; i<siz; i++)
    {
        ll cur=v[i].first;
        //cout<<cur<<" "<<i<<": \n";
        for(auto x:v[i].second)
        {
            //cout<<x<<" ";
            dp[i]=max(max(dp[i], m[x]+cur), max(dp[i-1]+cur-c, dp[i-1]));
            m[x]=max(m[x]+cur, dp[i-1]+cur-c);
        }
        //cout<<endl;
    }
    cout<<dp[siz-1];

}