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#include <bits/stdc++.h>
#define int long long
using namespace std;
int n, c;
const int MX = 5e5 + 9;
vector<pair<int,int>>v;
int tre[4*MX];
bool onstack[MX];
void update(int node, int start, int end, int tar, int val){
    if(start == end) tre[node] = max(tre[node], val);
    else{
        int mid = (start + end)>>1;
        if(mid >= tar){
            update(node<<1, start, mid, tar, val);
        }
        else{
            update((node<<1)+1, mid+1, end, tar, val);
        }
        tre[node] = max(tre[node<<1], tre[(node<<1)+1]);
    }
}
int query(int node, int L, int R, int start, int end){
    if(start > R or end < L) return 0;
    else if(start >= L and end <= R) return tre[node];
    else{
        int mid = (start + end)>>1;
        return max(query(node<<1, L, R, start, mid), query((node<<1)+1, L, R, mid+1, end));
    }
}
int32_t main(){
    cin >> n >> c;
    for(int i = 0; i < n; ++i){
        int a, w; cin >> a >> w;
        v.push_back({a, w});
    }
    sort(v.begin(), v.end(), greater<pair<int,int>>());
    int curh = v[0].first;
    queue<pair<int,int>>q;
    for(int i = 0; i < n; ++i){
        int a = v[i].first, w = v[i].second;
        if(a < curh){
            while(!q.empty()){
                if(onstack[q.front().first]) update(1, 1, MX, q.front().first, q.front().second);
                onstack[q.front().first] = false;
                q.pop();
            }
        }
        curh = a;
        if(onstack[w]) continue;
        onstack[w] = true;
        int val = a+query(1, w, w, 1, MX);
        val = max(val, a+query(1, 1, w-1, 1, MX)-c);
        val = max(val, a+query(1, w+1, MX, 1, MX)-c);
        q.push({w, val});
    }
    while(!q.empty()){
        if(onstack[q.front().first]) update(1, 1, MX, q.front().first, q.front().second);
        onstack[q.front().first] = false;
        q.pop();
    }
    cout << query(1, 1, MX, 1, MX);
}