import sys
sys.setrecursionlimit(10 ** 6)

def find_path(now):
    if passed[now] < 1:
        print(now + 1)
        for i in range(len(label[now])):
            if not visited[label[now][i][1]]:
                visited[label[now][i][1]] = True
                find_path(label[now][i][0])
                print(now + 1)
        passed[now] += 1

n, m = map(int, input().split())
label = [[] for _ in range(n)]
visited = [False] * m
passed = [0] * n

for i in range(m):
    x, y = map(int, input().split())
    label[x - 1] += [[y - 1, i]]
    label[y - 1] += [[x - 1, i]]


m *= 2
print(m)
find_path(0)
# print(1)