from collections import deque

N=int(input())
E={v:set() for v in range(1,N+1)}
F={}

for _ in range(N):
    a,b=map(int,input().split())
    E[a].add(b)
    E[b].add(a)
    F[(a,b)]=False


T=[False]*(N+1)

S=deque([1])
T[1]=True
while S:
    v=S.pop()
    for u in E[v]:
        if not T[u]:
            S.append(u)
            T[u]=True
            a,b=min(u,v),max(u,v)
            F[(a,b)]=True

p,q=min(F,key=lambda x:F[x])
E[p].remove(q)
E[q].remove(p)

T=[0]*(N+1)
D=[0]*(N+1)
S=deque([p])
T[p]=float("inf")

while S:
    v=S.pop()
    for u in E[v]:
        if not T[u]:
            S.append(u)
            D[u]=D[v]+1
            T[u]=v

H=D[q]
P=[0]*(H+1)
P[0]=q

y=q
i=0
while y!=p:
    i+=1
    y=T[y]
    P[i]=y

print(H+1)
X=[(p,q)]+[(P[i],P[i+1]) for i in range(H)]
print("\n".join(map(lambda x:"{} {}".format(x[0],x[1]),X)))