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

int main(void) {
    ll N,D,T,K;
    cin >> N >> D;
    ll to[N],ky[N];
    cin >> T >> K;
    to[0] = T; ky[0] = K - D;
    for (int i=1; i<N; i++) {
        cin >> T >> K;
        to[i] = max(to[i-1] + T, ky[i-1] + T - D);
        ky[i] = max(to[i-1] + K - D, ky[i-1] + K);
    }
    cout << max(to[N-1], ky[N-1]) << endl;
    return 0;
}