#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
    int s, n;
    cin >> s >> n;
    vector <int> a(n);
    for(int i = 0; i < n; i++)
        cin >> a[i];
    sort(a.begin(), a.end());
    int prev_s = 0, count = 0;
    for(int i = 0; i < n; i++)
        if(a[i] >= s)
        {
            prev_s = a[i];
            count++;
            for(int j = i; j < n; j++)
                if(a[j] - prev_s >= 3)
                {
                    count++;
                    prev_s = a[j];
                }
            break;
        }
    cout << count;
    return 0;
}