#include<iostream>
#include<vector>
#include<algorithm>
#include<functional>
#include<set>
#include<map>
#include<string>
#include<iomanip>
#include<cmath>
#include<stack>
#include<tuple>

int main()
{
	int W, H;
	std::cin >> W >> H;
	int field[102][102] = {};
	std::vector<std::pair<int, int>> start[1001];
	for (int i = 1; i <= H; ++i)
	{
		for (int k = 1; k <= W; ++k)
		{
			std::cin >> field[i][k];
			start[field[i][k]].emplace_back(i, k);
		}
	}
	for (int i = 1; i <= 1000; ++i)
	{
		bool check[102][102] = {};
		for (auto[a, b] : start[i])
		{
			if (check[a][b])
			{
				continue;
			}
			std::stack<std::tuple<int, int, int, int>> stack;
			stack.emplace(a, b, 0, 0);
			while (stack.size())
			{
				auto[s, t, ps, pt] = stack.top();
				stack.pop();
				if (check[s][t])
				{
					std::cout << "possible" << std::endl;
					return 0;
				}
				check[s][t] = true;
				{
					auto ns = s + 1;
					auto nt = t;
					if (field[ns][nt] == field[s][t] && (ns != ps || nt != pt))
					{
						stack.emplace(ns, nt, s, t);
					}
				}
				{
					auto ns = s - 1;
					auto nt = t;
					if (field[ns][nt] == field[s][t] && (ns != ps || nt != pt))
					{
						stack.emplace(ns, nt, s, t);
					}
				}
				{
					auto ns = s;
					auto nt = t + 1;
					if (field[ns][nt] == field[s][t] && (ns != ps || nt != pt))
					{
						stack.emplace(ns, nt, s, t);
					}
				}
				{
					auto ns = s;
					auto nt = t - 1;
					if (field[ns][nt] == field[s][t] && (ns != ps || nt != pt))
					{
						stack.emplace(ns, nt, s, t);
					}
				}
			}
		}
	}
	std::cout << "impossible" << std::endl;
}