import sys, math
sys.setrecursionlimit(10**5+1)

def input():
    return sys.stdin.readline()[:-1]

def dfs(graph,ima,dist):
    for tsugi in graph[ima]:
        if dist[tsugi] > 10*5:
            dist[tsugi] = dist[ima]+1
            dfs(graph,tsugi,dist)

def main():
    N, K = map(int,input().split())
    G = [[] for k in range(N)]
    for k in range(N-1):
        a, b = map(int,input().split())
        G[a-1].append(b-1)
        G[b-1].append(a-1)

    if N < K:
        print(-1)
    else:
        print(K-1)

if __name__ == '__main__':
    main()