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#include <bits/stdc++.h>
using namespace std;
using i64 = long long;

int main() {
    int n, cost;
    cin >> n >> cost;

    vector<int> hei(n), col(n);
    for (int i = 0; i < n; i++) {
        cin >> hei[i] >> col[i];
    }

    set<pair<i64,int>> pairs;
    map<int,i64> dp;

    auto consider = [&](int x, i64 v) {
        if (dp[x] >= v) return;
        pairs.erase({dp[x], x});
        dp[x] = v;
        pairs.insert({v, x});
    };

    consider(0, 0);
    for (int i = 0; i < n; i++) {
        if (i == 0 || hei[i] > hei[i-1]) {
            set<int> xs;
            for (int j = i; j < n && hei[j] == hei[i]; j++) {
                xs.insert(col[j]);
            }

            i64 best = pairs.empty() ? 0 : rbegin(pairs)->first;
            for (int x: xs) {
                consider(x, hei[i] + max(dp[x], best-cost));
            }
        }
    }
    cout << rbegin(pairs)->first << '\n';
}