#include <bits/stdc++.h>
using namespace std;
int main()
{
	long long p[1000005], n, m;
	int sum = 0;
	cin >> n >> m;
	for (int i = 1;i <= n;i ++)
	{
		int x;
		cin >> x;
		if (x-2 >= 1) p[x-2] = 1;
		if (x-1 >= 1) p[x-1] = 1;
        p [x] = 1;
		if (x+1 <= n) p[x+1] = 1;
		if (x+2 <= n) p[x+2] = 1;
	}
	for (int i = 1;i <= n;i ++)
	{
		sum += p[i];
	}
	cout << sum << "\n";
	return 0;
}

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