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

const ll INF = 1LL << 60;
ll dp[101][2];

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll N, D;
	cin >> N >> D;

	fill((ll*)begin(dp), (ll*)end(dp), -INF);
	dp[0][0] = 0;
	for(int i = 0; i < N; i++) {
		ll a[2];
		cin >> a[0] >> a[1];
		for(int j = 0; j < 2; j++) {
			if(dp[i][j] == -INF) continue;
			for(int k = 0; k < 2; k++) {
				ll sum = a[k];
				if(j != k) sum -= D;
				dp[i + 1][k] = max(dp[i + 1][k], dp[i][j] + sum);
			}
		}
	}
	cout << max(dp[N][0], dp[N][1]) << endl;
}