#include "bits/stdc++.h"

using namespace std;
typedef long long ll;
const ll MOD = 1e9 + 7;
int dp[101][2];
int v[100][2];
int main() {
	int N, D;
	cin >> N >> D;
	for (int i = 0; i < N; i++) {
		cin >> v[i][0] >> v[i][1];
	}
	fill((int*)dp, (int*)(dp + 101), -(1 << 30));
	dp[0][0] = 0;
	for (int i = 0; i < N; i++) {
		for (int j = 0; j < 2; j++) {
			for (int k = 0; k < 2; k++) {
				int x = dp[i][j] + v[i][k];
				if (j != k) x -= D;
				dp[i + 1][k] = max(dp[i + 1][k], x);
			}
		}
	}
	cout << max(dp[N][0], dp[N][1]) << endl;
}