from collections import Counter
pairs = [(int(x[0]), int(x[1])) for x in [input().split() for _ in range(int(input()))]]
xs, ys = [x for x, y in pairs], [y for x, y in pairs]
goal = max(xs) + min(xs), max(ys) + min(ys)
c = Counter(pairs)
print("impossible"
      if any(((p[0] * 2, p[1] * 2) == goal and c[p] % 2) or (c[p] != c[(goal[0] - p[0], goal[1] - p[1])]) for p in c)
      else "possible")