#include <iostream>

int main() {
    using namespace std;
    int n, d;
    cin >> n >> d;
    int t[n], k[n];
    for (int i = 0; i < n; i++) {
        cin >> t[i] >> k[i];
    }

    int pt = t[0];
    int pk = k[0] - d;
    for (int i = 1; i < n; i++) {
        int ptt = pt + t[i];
        int pkt = pk + t[i] - d;
        int pkk = pk + k[i];
        int ptk = pt + k[i] - d;
        pt = ptt > pkt ? ptt : pkt; 
        pk = pkk > ptk ? pkk : ptk;
    }
    int result = pt > pk ? pt : pk;
    cout << result << endl;
}