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#include <bits/stdc++.h>
 
using namespace std;
 
#define endl '\n'
#define L long long
#define MP make_pair
#define REP(i, n) for(int i = 0; i < n; ++i)
#define REPR(i, n) for(int i = n - 1; i >= 0; --i)
#define FOR(i, a, b) for(int i = a; i < b; ++i)
#define FORR(i, a, b) for(int i = b - 1; i >= a; --i)
#define EB emplace_back
#define ST first
#define ND second
#define S size
#define RS resize
 
template<class T> using P = pair<T, T>;
template<class T> using V = vector<T>;
 
int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int n;
    L c;
    cin >> n >> c;
    int d = 500001;

    V<V<int>> blocks(d);
    REP(i, n) {
        int a, w;
        cin >> a >> w;
        blocks[a].EB(w);
    }

    V<L> res(d);
    set<pair<L, int>> res_s;
    REP(i, d) {
        res_s.insert(MP(0LL, 0));
    }

    REP(a, d) {
        V<pair<L, int>> new_res, old_res;
        for (int w : blocks[a]) {
            L r = max(res[w], prev(res_s.end())->ST - c) + a;
            new_res.EB(MP(r, w));
            old_res.EB(MP(res[w], w));
        }
        REP(i, int(new_res.size())) {
            auto& [r, w] = new_res[i];
            res[w] = r;
            res_s.erase(old_res[i]);
            res_s.insert(MP(r, w));
        }
    }

    cout << prev(res_s.end())->ST << '\n';
}

// g++ -std=c++20 -Wall -Wextra -Wshadow -Wconversion -D_GLIBCXX_DEBUG -O3 wie.cpp -o wie