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

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


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)

stack = [(0,-1)]

while len(stack) > 0:
    u = stack[len(stack)-1][0]
    p = stack[len(stack)-1][1]
    stack.pop()
    if len(g[u]) == 1:
        l.append(u)
    for v in g[u]:
        if v != p:
            stack.append((v,u))
    

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