n = int(input())
if n % 2: print("impossible"), exit()
t, a = n // 2, sorted(int(input().split()[0]) for _ in range(n))  # `t` is the number of teams
# `b` is the exact skill per team. In case any of them is fractional, "impossible" is printed.
b = sum(a) / t
print(("im" if any(b1 + b2 != b for b1, b2 in zip(a, a[::-1])) else "") + "possible")