#!/usr/bin/python3

n, r = [int(x) for x in input().split()]
topics = [[(int(x) - 1)//n for x in input().split()] for _ in range(r)]
students = [[topics[i][j] for i in range(r)] for j in range(2*n)]
flipped = sorted([s[1:] if s[0] == 0 else [1 - x for x in s[1:]] for s in students])
print("possible" if all(flipped[i] == flipped[i+1] for i in range(0, 2*n, 2)) else "impossible")