/* {{{ Shinobu kawaii */
/*
  ______   __        _                  __                 
.' ____ \ [  |      (_)                [  |                
| (___ \_| | |--.   __   _ .--.   .--.  | |.--.   __   _   
 _.____`.  | .-. | [  | [ `.-. |/ .'`\ \| '/'`\ \[  | | |  
| \____) | | | | |  | |  | | | || \__. ||  \__/ | | \_/ |, 
 \______.'[___]|__][___][___||__]'.__.'[__;.__.'  '.__.'_/ 

*/
// clang-format off
/* }}} */

#include <bits/stdc++.h>
using namespace std;

// #define int long long
struct Fast {Fast(){std::cin.tie(0);ios::sync_with_stdio(false);}} fast;

/* cpp template {{{ */

/* short */
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define Fi first
#define Se second
#define ALL(v) (v).begin(), (v).end()
#define X real()
#define Y imag()

/* REPmacro */
#define REPS(i, a, n) for (auto i = (a); i < (n); ++i)
#define REP(i, n) REPS(i, 0, n)
#define RREP(i, n) REPS(i, 1, n + 1)
#define DEPS(i, a, n) for (auto i = (a); i >= n; --i)
#define DEP(i, n) DEPS(i, n, 0)

/* debug */
#define debug(x) cerr << x << " " << "(L:" << __LINE__ << ")" << '\n';

/* alias */
using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using pii = pair<int, int>;
using D = double;
using P = complex<D>;
template <typename T> using PQ = priority_queue<T>;
template <typename T> using minPQ = priority_queue<T, vector<T>, greater<T>>;

/* const */
const int INF = 1001001001;
const ll LINF = 1001001001001001001ll;
const int MOD = 1e9 + 7;
const D EPS = 1e-9;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1}, dy[] = {1, 0, -1, 0, 1, -1, -1, 1};

/* func */
inline bool inside(int y, int x, int H, int W) {return y >= 0 && x >= 0 && y < H && x < W;}
inline int in() {int x; cin >> x; return x;}
inline ll IN() {ll x; cin >> x; return x;}
template <typename T> inline bool chmin(T& a, const T& b) {if (a > b) a = b; return a > b;}
template <typename T> inline bool chmax(T& a, const T& b) {if (a < b) a = b; return a < b;}
template <typename T> inline void print(const T& x) {cout << x << '\n';}
template <typename T, typename S> inline void print(const pair<T, S>& p) {cout << p.first << " " << p.second << '\n';}
template <typename T, typename S> inline void print(const vector<pair<T, S>>& v) {
  REP(i, v.size()) print(v[i]);
}
template <typename T> inline void print(const vector<T>& v, string s = " ") {
  REP(i, v.size()) {
    if (i != 0) cout << s;
    cout << v[i];
  }
  cout << '\n';
}
// clang-format on
/* }}} */

signed main() {
  int n, k;
  while (cin >> n >> k, n || k) {
    int ans = -INF;
    bool white = false;
    vi v, shinobu(n + 5, 0);
    REP(i, k) {
      int t = in();
      if (t == 0) {
        white = true;
        continue;
      }
      v.eb(t);
    }
    sort(ALL(v));
    shinobu[v[0]] = 1;
    RREP(i, v.size() - 1) {
      if (v[i] - v[i - 1] == 1) {
        shinobu[v[i]] = shinobu[v[i - 1]] + 1;
      } else {
        shinobu[v[i]] = 1;
      }
      chmax(ans, shinobu[v[i]]);
    }
    if (white) {
      RREP(i, n + 1) {
        if (shinobu[i]) continue;
        int t = 0;
        if (shinobu[i - 1]) {
          chmax(ans, shinobu[i - 1] + 1);
          t = 1;
        }
        if (shinobu[i + 1]) {
          int idx = i - 1;
          ++i;
          for (; shinobu[i] > 0; ++i);
          chmax(ans, shinobu[idx] + shinobu[i - 1] + t);
          --i;
        }
      }
    }
    print(ans);
  }

  return 0;
}

Measured memory_bytes: 3900