102

# -*- coding: utf-8 -*-

def q():
    print(-1)
    exit()

answer = list()

def fleury(u, n, m):
    while cnt[u]:
        for i in range(n[0]):
            if g[u][i] > 0:
                v = i
                break
        m[0] -= 1
        g[u][v] -= 1
        if u != v:
            g[v][u] -= 1
        cnt[u] -= 1
        cnt[v] -= 1
        fleury(v, n, m)
    answer.append(u + 1)


if __name__ == '__main__':
    n, m = map(int, input().split())
    g = [[0 for x in range(n)] for y in range(n)]
    cnt = [0 for i in range(n)]

    for i in range(m):
        x, y = map(int, input().split())
        x -= 1
        y -= 1
        if i == 0:
            start = x
        g[x][y] += 1
        if x != y:
            g[y][x] += 1
        cnt[x] += 1
        cnt[y] += 1

    v1 = -1
    v2 = -1
    for i in range(n):
        if cnt[i] % 2 == 1:
            if v1 == -1:
                v1 = i
                start = i
            elif v2 == -1:
                v2 = i
            else:
                q()

    if v1 == -1:
        v1 = start
    m = [m]
    n = [n]
    fleury(v1, n, m)
    cnt = []
    if m[0] != 0:
        q()
    print(len(answer))
    print(' '.join(map(str, answer)))