#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pii;

const int N = 5e5 + 2;
ll DP[N];
vector<pii> blocks;
queue<pii> buff;

void Print(int n){
    for(int i = 0 ; i < n ; i++) cerr << DP[i] << " ";
    cerr << '\n';
}

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(0);
    ll n, c, a, w, best = 0;
    cin >> n >> c;
    for(int i = 0 ; i < n ; i++){
        cin >> a >> w;
        blocks.push_back({a, w});
    }
    blocks.push_back({-1, -1});
    for(int i = n - 1 ; i >= 0 ; i--){
        if(blocks[i].first != blocks[i + 1].first){
            while(!buff.empty()){
                DP[buff.front().second] = buff.front().first;
                best = max(buff.front().first, best);
                buff.pop();
            }
        }
        a = blocks[i].first;
        w = blocks[i].second;
        buff.push({max(DP[w], best - c) + a, w});
        //Print(n);
    }
    while(!buff.empty()){
        DP[buff.front().second] = buff.front().first;
        best = max(buff.front().first, best);
        buff.pop();
    }
    cout << best << '\n';
}