#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
ll h[100001];

int n, d;
int ti[110], ki[110];
int dpti[110], dpki[110];
int INF = 987654321;

int main() {
    ios::sync_with_stdio(false);
    cin >> n >> d;
    for (int i = 1; i <= n; i++) {
        cin >> ti[i] >> ki[i];
    }
    dpti[0] = 0;
    dpki[0] = -INF;

    for (int i = 1; i <=n; i++) {
        dpti[i] = max(dpti[i-1], dpki[i-1] - d) + ti[i];
        dpki[i] = max(dpki[i-1], dpti[i-1] - d) + ki[i];
    }
    cout << max(dpti[n], dpki[n]) << endl;



    return 0;
}