// yukicoder: No.13 囲みたい！
// 2019.5.26 bal4u

#include <stdio.h>

#define gc() getchar_unlocked()
#define pc(c) putchar_unlocked(c)
int in() {    // 非負整数の入力
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

void outs(char *s) { while (*s) pc(*s++); pc('\n'); }

typedef struct { int r, c, pr, pc; } Q;
Q q[10005]; int top;
char vis[103][103];
int W, H;
int map[103][103];
int mv[4][2] = {{-1,0}, {0,1}, {1,0}, {0,-1}};

int dfs(int rr, int cc, int val) {
	int i, r, c, pr, pc;
	
	q[0].r = rr, q[0].c = cc, pr = -1, top = 1;
	while (top) {
		r = q[--top].r, c = q[top].c, pr = q[top].pr, pc = q[top].pc;
		if (vis[r][c]) return 1;
		vis[r][c] = 1;
		for (i = 0; i < 4; i++) {
			rr = r + mv[i][0], cc = c + mv[i][1];
			if (rr < 0 || rr >= H || cc < 0 || cc >= W) continue;
			if (rr == pr && cc == pc) continue;
			if (map[rr][cc] == val) {
				q[top].r = rr, q[top].c = cc, q[top].pr = r, q[top++].pc = c;
			}
		}
	}
	return 0;
}

int main()
{
	int r, c;

	W = in(), H = in();
	if (W == 1 || H == 1) { outs("impossible"); return 0; }
	for (r = 0; r < H; r++) for (c = 0; c < W; c++) map[r][c] = in();
	for (r = 0; r < H; r++) for (c = 0; c < W; c++) if (!vis[r][c]) {
		if (dfs(r, c, map[r][c])) { outs("possible"); return 0; }
	}
	outs("impossible");
	return 0;
}