n = int(input())
t, a = n // 2, sorted(tuple(map(int, input().split())) for _ in range(n))  # `t` is the number of teams
# `b` and `p` are the exact skill per team. In case any of them is fractional, "impossible" is printed.
b, p = (sum(c[i] for c in a) / t for i in range(2))
print(("im" if any(b1 + b2 != b or p1 + p2 != p for (b1, p1), (b2, p2) in zip(a, a[::-1])) else "") + "possible")