#include <iostream>

using namespace std;

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

    int t, k;
    cin >> t >> k;
    k -= d;
    for (int i = 1; i < n; i++) {
        int a, b;
        cin >> a >> b;
        a += max(t, k - d), b += max(t - d, k);
        t = a, k = b;
    }

    cout << max(t, k) << endl;
    return 0;
}