N, K = map(int, input().split())
S1 = set(i+1 for i in range(4))
S2 = set()
Ans = '-1'

if N >= K:
    for _ in range(N-1):
        a, b = map(int, input().split())
        S2 |= {a}
        S2 |= {b}
        if len(S1 - set(S2)) == 0:
            Ans = K-1

print(Ans)