#include "bits/stdc++.h"
using namespace std;

int main() {
	long long N, D, T[100], K[100], DP[110][2] = {};
	DP[0][1] = -1000000000;
	cin >> N >> D;
	for (int i = 0; i < N; i++) cin >> T[i] >> K[i];
	for (int i = 1; i <= N; i++) {
		DP[i][0] = max(DP[i - 1][0], DP[i - 1][1] - D) + T[i - 1];
		DP[i][1] = max(DP[i - 1][1], DP[i - 1][0] - D) + K[i - 1];
	}
	cout << max(DP[N][0], DP[N][1]) << endl;
}