#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
#include <iterator>
#include <set>
using namespace std;

int main() {
    int customer_size, n;
    cin >> customer_size >> n;
    set <int> shoes;
    for (int i = 0; i < n; i++) {
        int current_shoes;
        cin >> current_shoes;
        shoes.insert(current_shoes);
    }
    int correct_shoes = 0;
    for (auto current_shoes : shoes) {
        if (current_shoes - customer_size >= 0 && correct_shoes == 0) {
            correct_shoes++;
            customer_size = current_shoes;
        }
        if (current_shoes - customer_size >= 3) {
            correct_shoes++;
            customer_size = current_shoes;
        }
    }
    cout << correct_shoes;
}