#!/usr/bin/python3
import sys
sys.setrecursionlimit(10**9)
n, p = [int(x) for x in sys.stdin.readline().split()]

g = [[] for _ in range(n)]
l = []

def dfs(u, p):
    if len(g[u]) == 1:
        l.append(u)
    for v in g[u]:
        if v != p:
            dfs(v, u)


for i in range(n-1):
    u, v = [int(x) for x in sys.stdin.readline().split()]
    g[u].append(v)
    g[v].append(u)

dfs(0, -1)

ans = (len(l)+1)//2
print(ans)
for i in range(ans):
    print(l[i], l[(i+ans)%len(l)])