#include <cstdio>
#include <vector>
#define rep(i,n) for(int i = 0; i < n; i++)
#define pb push_back
using namespace std;

int n, k;
bool num[100000];

int main(){
	while(scanf("%d%d", &n, &k),n||k){
		rep(i,100000) num[i] = false;
		bool ch = false;
		rep(i,k){
			int tmp; scanf("%d", &tmp);
			if(tmp == 0){
				ch = true;
				continue;
			}
			tmp--;
			num[tmp] = true;
		}
		int ans = 0;
		if(!ch){
			int cnt = 0;
			rep(i,n){
				if(!num[i]){
					if(ans < cnt) ans = cnt;
					cnt = 0;
					continue;
				} else cnt++;
			}
			printf("%d\n", ans);
			continue;
		}
		vector<int> v;
		int cnt = 0;
		rep(i,n){
			if(!num[i]){
				if(cnt > 0) v.pb(cnt);
				cnt = 0;
				v.pb(0);
			} else{
				cnt++;
			}
		}
		if(cnt > 0) v.pb(cnt);
		rep(i,v.size()){
			int tmp = 1;
			if(v[i] > 0){
				if(ans < v[i]) ans = v[i];
			} else{
				if(i != 0) tmp += v[i-1];
				if(i != v.size()-1) tmp += v[i+1];
				if(ans < tmp) ans = tmp;
			}
		}
		printf("%d\n", ans);
	}
}