#include "bits/stdc++.h"
#include<unordered_map>
#include<unordered_set>
#pragma warning(disable:4996)
using namespace std;
using ld = long double;
template<class T>
using Table = vector<vector<T>>;
const ld eps=1e-9;

// < "D:\D_Download\Visual Studio 2015\Projects\programing_contest_c++\Debug\a.txt"

typedef int Weight;
struct Edge {
	int src, dst;
	Weight weight;
	Edge(int src, int dst, Weight weight) :
		src(src), dst(dst), weight(weight) { }
};
struct Dscc {
public:

	//belongs,newedges
	pair<vector<vector<int>>, vector<vector<int>>>get(const vector<vector<int>>&edges) {
		nums.resize(edges.size());
		fill(nums.begin(), nums.end(), -1);
		vector<vector<int>>revedges(edges.size());
		for (int i = 0; i < edges.size(); ++i) {
			for (auto j : edges[i]) {
				revedges[j].push_back(i);
			}
		}
		int num = 0;
		for (int i = 0; i < edges.size(); ++i) {
			dfs(i, num, edges);
		}
		vector<int>big(nums.size());
		for (int i = 0; i < nums.size(); ++i) {
			big[nums[i]] = i;
		}
		reverse(big.begin(), big.end());
		unis.resize(edges.size());
		fill(unis.begin(), unis.end(), -1);
		num = 0;
		for (int i = 0; i < big.size(); ++i) {

			dfs2(big[i], num, revedges);
			num++;
		}
		vector<int>nums;
		for (int i = 0; i < unis.size(); ++i) {
			nums.push_back(unis[i]);
		}
		sort(nums.begin(), nums.end());
		nums.erase(unique(nums.begin(), nums.end()), nums.end());


		map<int, int>mp;
		for (int i = 0; i < nums.size(); ++i) {
			mp[nums[i]] = i;
		}
		for (int i = 0; i < unis.size(); ++i) {
			unis[i] = mp[unis[i]];
		}

		vector<vector<int>>belongs(nums.size());
		for (int i = 0; i < unis.size(); ++i) {
			belongs[unis[i]].push_back(i);
		}
		vector<vector<int>>newedges(nums.size());
		for (int i = 0; i < edges.size(); ++i) {
			for (auto j : edges[i]) {
				if (unis[i] != unis[j]) {
					newedges[unis[i]].push_back(unis[j]);
				}
			}
		}
		return make_pair(belongs, newedges);
	}

	//belongs,newedges
	pair<vector<vector<int>>, vector<vector<Edge>>>get(const vector<vector<Edge>>&edges) {
		const int old_node_size = edges.size();
		nums.resize(old_node_size);
		fill(nums.begin(), nums.end(), -1);
		vector<vector<Edge>>revedges(old_node_size);
		for (const auto& e : edges) {
			for (const auto& j : e) {
				revedges[j.dst].push_back(Edge(j.dst, j.src, j.weight));
			}
		}
		int num = 0;
		for (int i = 0; i < old_node_size; ++i) {
			dfs(i, num, edges);
		}
		vector<int>big(old_node_size);
		for (int i = 0; i < old_node_size; ++i) {
			big[nums[i]] = i;
		}
		reverse(big.begin(), big.end());

		unis.resize(old_node_size);
		fill(unis.begin(), unis.end(), -1);

		num = 0;
		for (int i = 0; i < old_node_size; ++i) {
			dfs2(big[i], num, revedges);
			num++;
		}
		vector<int>nums;
		for (int i = 0; i < old_node_size; ++i) {
			nums.push_back(unis[i]);
		}

		sort(nums.begin(), nums.end());
		nums.erase(unique(nums.begin(), nums.end()), nums.end());
		const int new_node_size = nums.size();

		map<int, int>mp;
		for (int i = 0; i < new_node_size; ++i) {
			mp[nums[i]] = i;
		}
		for (int i = 0; i < old_node_size; ++i) {
			unis[i] = mp[unis[i]];
		}

		vector<vector<int>>belongs(new_node_size);
		for (int i = 0; i < old_node_size; ++i) {
			belongs[unis[i]].push_back(i);
		}
		vector<vector<Edge>>newedges(new_node_size);
		for (const auto& e : edges) {
			for (const auto& j : e) {
				if (unis[j.src] != unis[j.dst]) {
					newedges[unis[j.src]].push_back(Edge(unis[j.src], unis[j.dst], j.weight));
				}
			}
		}
		return make_pair(belongs, newedges);
	}
private:
	vector<int>nums;
	vector<int>unis;

	void dfs(const int id, int &num, const vector<vector<int>>&edges) {
		if (nums[id] != -1)return;
		else {
			nums[id] = -2;
			for (auto i : edges[id]) {
				dfs(i, num, edges);
			}
		}
		nums[id] = num++;
		return;
	}
	void dfs2(const int id, const int &num, const vector<vector<int>>&edges) {
		if (unis[id] != -1)return;
		else {
			unis[id] = -2;
			for (auto i : edges[id])
				dfs2(i, num, edges);
		}
		unis[id] = num;
		return;
	}
	void dfs(const int id, int &num, const vector<vector<Edge>>&edges) {
		if (nums[id] != -1)return;
		else {
			nums[id] = -2;
			for (auto i : edges[id]) {
				dfs(i.dst, num, edges);
			}
		}
		nums[id] = num++;
		return;
	}
	void dfs2(const int id, const int &num, const vector<vector<Edge>>&edges) {
		if (unis[id] != -1)return;
		else {
			unis[id] = -2;
			for (auto i : edges[id]) {
				dfs2(i.dst, num, edges);
			}
		}
		unis[id] = num;
		return;
	}

}dscc;
int come[300];
bool dfs(const int now,const vector<vector<int>>&edges, const int goal) {
	if (come[now])return true;
	come[now] = true;
	if (now == goal)return true;
	else {
		if (edges[now].empty())return false;
		for (auto e : edges[now]) {
			if (e == now)assert(false);
			if (!dfs(e, edges, goal))return false;
		}
		return true;
	}
}
int main() {
	while (1) {
		memset(come, false, sizeof(come));
		int amax;
		cin >> amax;
		if (!amax)break;
		int N; cin >> N;
		const int goal = N + 1;
		const int start = 0;
		vector<int>ds(goal+1);
		for (int i = 0; i < N; ++i) {
			cin >> ds[i+1];
		}
		vector<vector<int>>edges(N + 2);
		for (int i = 0; i < N + 1; ++i) {
			for (int j = 1; j <= amax; ++j) {
				int next;
				if (i + j >= goal) {
					next = goal;
				}
				else {
					if (i + j + ds[i + j] >= goal) {
						next = goal;
					}
					else if (i + j + ds[i+j]<0) {
						next = 0;
					}
					else {
						next = i + j + ds[i + j];
					}
				}
				if (find(edges[i].begin(), edges[i].end(), next) == edges[i].end()) {
					edges[i].push_back(next);
				}
				sort(edges[i].begin(), edges[i].end());
			}
		}
		auto p=dscc.get(edges);
		int newstart=-1;
		int newgoal=-1;
		for (int i = 0; i < p.first.size();++i) {
			auto b = p.first[i];
			if (find(b.begin(), b.end(), 0)!=b.end()) {
				newstart = i;
			}
			if (find(b.begin(), b.end(), goal) != b.end()) {
				newgoal= i;
			}
		}
		if (newstart == -1 || newgoal == -1)cout << "ff";
		auto newedges = p.second;
		
		bool ans = dfs(newstart, newedges, newgoal);
		if (ans) {
			cout << "OK" << endl;
		}
		else {
			cout << "NG" << endl;
		}
	}
	return 0;
}

Measured memory_bytes: 3156