#include <bits/stdc++.h>

const int N = 10010;
int n, l, ans;
int a[N];

bool valid(int t) {
	int x = 0, y = 0;
	for (int j = 1; a[j] < t; ++ j) x = j;
	for (int j = n; a[j] > t; -- j) y = j;
	while (x >= 1 && y <= n)
		if (a[x --] + a[y ++] != 2 * t) return false;
	return true;
}

int main() {
	freopen("device.in", "r", stdin);
	freopen("device.out", "w", stdout);
	std::cin >> n >> l, l += l;
	for (int i = 1; i <= n; ++ i)
		std::cin >> a[i], a[i] += a[i];
	std::sort(a + 1, a + 1 + n);
	for (int i = 1; i <= l; ++ i) 
		ans += valid(i);
	std::cout << ans << '\n';
	return 0;
}