// No.561 東京と京都
// https://yukicoder.me/problems/no/561
//

#include <iostream>
using namespace std;

int solve(int N, int D);


int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    int N, D;
    cin >> N >> D;
    int ans = solve(N, D);
    cout << ans << endl;
}

int solve(int N, int D) {
    int last_tokyo = 0;
    int last_kyoto = -D;
    
    for (auto i = 0; i < N; ++i) {
        int t, k;
        cin >> t >> k;
        int t_tokyo = max(last_tokyo + t, last_kyoto + t - D);
        int t_kyoto = max(last_kyoto + k, last_tokyo + k - D);
        last_tokyo = t_tokyo;
        last_kyoto = t_kyoto;
    }
    return max(last_tokyo, last_kyoto);
}