//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//  MMMMMMMMMMMMMMMMNOtXtdMMMMMNxzZMMNmgHyVVVVVW%..?/(.,(x-..........(HfffpffpffpffWHHffpffppfVyyWMMMMMM


//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

int main() {

    while (true) {

        int n, k;
        cin >> n >> k;

        if (!(n | k)) break;

        vector<int> a(k);
        for (int i = 0; i < k; i++) cin >> a[i];
        sort(a.begin(), a.end());

        bool zero = false;
        if (a[0] == 0) {
            zero = true;
            a.erase(a.begin());
            k--;
        }

        vector<pair<int, int>> edges;
        for (int i = 0, j = 0; i < k; i = j) {
            int len = 0;
            while (j < k && a[i] + len == a[j]) {
                j++;
                len++;
            }
            edges.push_back(make_pair(a[i], len));
        }

        int ans = 0;
        for (int i = 0; i < edges.size(); i++) ans = max(ans, edges[i].second + zero);

        if (zero) {
            for (int i = 0; i < edges.size() - 1; i++) {
                int to = edges[i].first + edges[i].second + 1;
                if (edges[i + 1].first == to) {
                    ans = max(ans, edges[i].second + edges[i + 1].second + 1);
                }
            }
        }

        cout << ans << endl;

    }

    return 0;
}






































Measured memory_bytes: 3356