class Graph:
    #入力定義
    def __init__(self,vertex=[]):
        self.vertex=list(vertex)
        self.edge_number=0
        self.adjacent={v:set() for v in vertex}

    #頂点の追加
    def add_vertex(self,*adder):
        k=len(self.vertex)
        m=0

        for u in adder:
            if u not in self.adjacent:
                self.adjacent[u]=set()
                self.vertex.append(u)

    #辺の追加
    def add_edge(self,From,To):
        for w in [From,To]:
            if w not in self.adjacent:
                self.add_vertex(w)

        if To not in self.adjacent[From]:
            self.adjacent[From].add(To)
            self.adjacent[To].add(From)
            self.edge_number+=1

    #辺を除く
    def remove_edge(self,u,v):
        for w in [u,v]:
            if w not in self.adjacent:
                self.add_vertex(w)

        if u in self.adjacent[v]:
            self.adjacent[u].remove(v)
            self.adjacent[v].remove(u)
            self.edge_number-=1

    #頂点を除く
    def remove_vertex(self,*v):
        for w in v:
            if w in self.adjacent:
                self.edge_number-=len(self.adjacent[w])
                for u in self.adjacent[w]:
                    self.adjacent[u].remove(w)
                del self.adjacent[w]


    #Walkの追加
    def add_walk(self,*walk):
        n=len(walk)
        for i in range(n-1):
            self.add_edge(walk[i],walk[i+1])

    #Cycleの追加
    def add_cycle(self,*cycle):
        self.add_walk(*cycle)
        self.add_edge(cycle[-1],cycle[0])

    #頂点の交換
    def __vertex_swap(self,p,q):
        self.vertex.sort()

    #グラフに頂点が存在するか否か
    def vertex_exist(self,v):
        return v in self.adjacent

    #グラフに辺が存在するか否か
    def edge_exist(self,u,v):
        if not(self.vertex_exist(u) and self.vertex_exist(v)):
            return False
        return u in self.adjacent[v]

    #近傍
    def neighbohood(self,v):
        if not self.vertex_exist(v):
            return []
        return list(self.adjacent[v])

    #次数
    def degree(self,v):
        if not self.vertex_exist(v):
            return 0

        return len(self.adjacent[v])

    #頂点数
    def vertex_count(self):
        return len(self.vertex)

    #辺数
    def edge_count(self):
        return self.edge_number

    #頂点vを含む連結成分
    def connected_component(self,v):
        if v not in self.adjacent:
            return []

        from collections import deque
        T={u:False for u in self.adjacent}
        T[v]=True
        S=deque([v])

        while S:
            u=S.popleft()
            for w in self.adjacent[u]:
                if not T[w]:
                    T[w]=True
                    S.append(w)

        return [x for x in self.adjacent if T[x]]

    #距離
    def distance(self,u,v):
        from collections import deque
        inf=float("inf")
        T={v:inf  for v in G.vertex}

        if u==v:
            return 0

        Q=deque([u])
        T[u]=0
        while Q:
            w=Q.popleft()
            for x in G.adjacent[w]:
                if T[x]==inf:
                    T[x]=T[w]+1
                    Q.append(x)
                    if x==v:
                        return T[x]
        return inf

    #最短路
    def shortest_path(self,u,v):
        from collections import deque
        inf=float("inf")
        T={v:[] for v in G.vertex}

        if u==v:
            return 0

        Q=deque([u])
        T[u]=[u]
        while Q:
            w=Q.popleft()
            for x in G.adjacent[w]:
                if not T[x]:
                    T[x]=T[w]+[x]
                    Q.append(x)
                    if x==v:
                        return T[x]
        return None

def DFS_Tree(G):
    from collections import deque
    T={v:False for v in G.vertex}
    v=G.vertex[0]
    D={v:float("inf") for v in G.vertex}

    S=deque([v])
    T[v]=True

    while S:
        v=S.pop()

        for u in G.adjacent[v]:
            if not T[u]:
                T[u]=True
                S.append(u)
                B.remove((min(u,v),max(u,v)))

#-------------------------------------------------
from collections import deque
N=int(input())
G=Graph(list(range(1,N+1)))
B=set()

for _ in range(N):
    u,v=map(int,input().split())
    G.add_edge(u,v)
    x,y=min(u,v),max(u,v)
    B.add((x,y))

DFS_Tree(G)
u,v=B.pop()
G.remove_edge(u,v)

P=G.shortest_path(u,v)+[u]
L=len(P)-1

X=[0]*L
for i in range(L):
    a,b=min(P[i],P[i+1]),max(P[i],P[i+1])
    X[i]=(a,b)

X.sort()
print(L)
print("\n".join(map(lambda x:"{} {}".format(x[0],x[1]),X)))