#ifndef LOCAL
#include <bits/stdc++.h>

using namespace std;

#define debug(...) (void(0))
#else
#include "algo/debug.h"
#endif

void solve() {
    int N, D; cin >> N >> D;
    pair<int64_t, int64_t> dp{0, -D};
    for(int i = 0; i < N; i++) {
        pair<int64_t, int64_t> ndp{-1e18, -1e18};
        int t, k; cin >> t >> k;
        ndp.first = max(ndp.first, dp.first + t);
        ndp.first = max(ndp.first, dp.second + k - D);
        ndp.second = max(ndp.second, dp.second + k);
        ndp.second = max(ndp.second, dp.first + t - D);
        dp.swap(ndp);
    }
    cout << max(dp.first, dp.second) << endl;
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int tt = 1;
    // std::cin >> tt;
    while (tt--) {
        solve();
    }
}