#include <iostream>
#include <string>

using namespace std;

int max(const int &a, const int &b) {
    return a > b ? a : b;
}

int main() {
    int day, cost;
    cin >> day >> cost;
    int wages[100][2];
    for (int i = 0; i < day; i++) {
        cin >> wages[i][0] >> wages[i][1];
    }

    int tokyo = wages[0][0];
    int kyoto = wages[0][1] - cost;

    for (int i = 1; i < day; i++) {
        int t = max(tokyo, kyoto - cost) + wages[i][0];
        int k = max(tokyo - cost, kyoto) + wages[i][1];

        tokyo = t;
        kyoto = k;
    }

    cout << max(tokyo, kyoto) << "\n";

    int a;
    cin >> a;
}