#!/usr/bin/env python3

def hit(x, y, k):  # Only consider pairs of rows (k%g and (k+r)%g). (where r = (y - x) % w)
    agrid, bgrid = [[False] * 4 for _ in range(4)], [[False] * 4 for _ in range(4)]
    for i in range(k, h, g): agrid[b[i][x]][b[i][y]], agrid[b[i][y]][b[i][x]] = True, True
    for i in range((k + (y - x) % w) % g, h, g): bgrid[b[i][x]][b[i][y]], bgrid[b[i][y]][b[i][x]] = True, True
    return any(agrid[i0][i1] and bgrid[i2][i3] for (i0, i1), (i2, i3) in zip(combinations, reversed(combinations)))


(h, w), combinations = map(int, input().split()), [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)]
b, g = [["wrgb".index(x) for x in input()] for _ in range(h)], __import__("math").gcd(h, w)
print("possible" if any(hit(x, y, k) for x in range(w) for y in range(w) for k in range(g)) else "impossible")