#include<bits/stdc++.h>
using namespace std;
const int MAXN = 5e5 + 5;
int n, m;
int dp[2 * MAXN];
int nxt[2 * MAXN];
int deg[2 * MAXN];
int side[2 * MAXN];
int match[2 * MAXN];
vector<int> adj[2 * MAXN];
void dfs(int u, int flag) {
if(side[u] != -1) return;
side[u] = flag;
for(int v : adj[u]) dfs(v, !flag);
}
int main(void) {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> m;
for(int i = 0, u, v;i < m;i++) {
cin >> u >> v;
u--, v--;
deg[u]++, deg[v]++;
adj[u].push_back(v);
adj[v].push_back(u);
}
memset(side, -1, sizeof side);
for(int i = 0;i < 2 * n;i++)
if(side[i] == -1) dfs(i, 0);
queue<int> q;
for(int i = 0;i < 2 * n;i++)
if(!side[i] && deg[i] == 1) q.push(i);
memset(nxt, -1, sizeof match);
memset(match, -1, sizeof match);
int ans = -1, s = -1;
while(q.size()) {
int u = q.front();
q.pop();
for(int v : adj[u])
if(match[v] == -1)
match[v] = u, match[u] = v;
for(int v : adj[match[u]]) {
if(match[v] == -1) {
if(dp[v] < dp[u] + 1)
dp[v] = dp[u] + 1, nxt[v] = u;
deg[v]--;
if(deg[v] == 1) q.push(v);
}
}
if(dp[u] > ans) ans = dp[u], s = u;
}
cout << 2 * (ans + 1) << "\n";
while(true) {
cout << match[s] + 1 << " " << s + 1;
s = nxt[s];
if(s == -1) break;
else cout << " ";
}
cout << "\n";
return 0;
}
