#!/usr/bin/env python3

# @EXPECTED_RESULTS@: WRONG_ANSWER, TIME_LIMIT_EXCEEDED
h, w = map(int, input().split())
p = [input() for _ in range(h)]
print((not any("".join(sorted([p[y][x], p[(Y := (y + s) % h)][x], p[y][(X := (x + s) % w)], p[Y][X]])) == "bgrw"
               for y in range(h) for x in range(w) for s in range(1, max(w, h)))) * "im" + "possible")