#!/usr/bin/env python3

C = {"w": 0, "r": 1, "g": 2, "b": 3}

h, w = map(int, input().split())
b = [[C[c] for c in input()] for _ in range(h)]
for x in range(w):
    for y in range(w):
        grid = [[False] * 4 for _ in range(4)]
        for i in range(h):
            grid[b[i][x]][b[i][y]] = True
            grid[b[i][y]][b[i][x]] = True
        if (
            (grid[0][1] and grid[2][3])
            or (grid[0][2] and grid[1][3])
            or (grid[0][3] and grid[1][2])
        ):
            print("possible")
            exit()
print("impossible")