#include<stdio.h>

int max(int a, int b) {
	int num;
	if (a > b) num = a;
	else num = b;
	return num;
}

int main(void) {
	int N,D;
	int T[1001],K[1001];
	long long int a[1001],b[1001];

	scanf("%d", &N);
	scanf("%d", &D);
	for (int i = 1; i <= N; i++) {
		scanf("%d", &T[i]);
		scanf("%d", &K[i]);
	}

	a[1] = T[1];
	b[1] = K[1] - D;
	
	for (int i = 2; i <= N; i++) {
		if (a[i - 1] > b[i - 1] - D) {
			a[i] = T[i] + a[i - 1];
		}
		else a[i] = T[i] - D + b[i - 1];

		if (b[i - 1] > a[i - 1] - D) {
			b[i] = K[i] + b[i - 1];
		}
		else b[i] = K[i] - D + a[i - 1];
	}

	printf("%d", max(a[N],b[N]));

	return 0;
	
}