#include <iostream>
#include <iomanip>
#include <vector>
#include <array>
#include <set>

using namespace std;

#define int long long

const long long INF = 1e18 + 7;
const int MAXN = 1e5 + 100;
const int N = 1e5 + 10;
const int MOD = 1e9 + 7;

int dp[MAXN][2];
vector<vector<int>> g;

void dfs(int v, int p) {
    dp[v][0] = dp[v][1] = 0;
    for (auto to: g[v]) {
        if (to == p) continue;
        dfs(to, v);
        dp[v][0] = max({dp[v][0], dp[to][0], dp[to][1]});
        dp[v][1] = max(dp[v][1], dp[to][0]);
    }
    dp[v][1] += 1;
}

void get_ans(int u, int p = -1) {
    for (auto v: g[u]) {
        if (v == p) {
            continue;
        }
        if (dp[v][1] > dp[v][0]) {
            cout << u + 1 << ' ' << v + 1 << '\n';
        }
        get_ans(v, u);
    }
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int n, m;
    cin >> n >> m;
    g.resize(n);
    for (int i = 0; i < m; ++i) {
        int u, v;
        cin >> u >> v;
        --u, --v;
        g[u].emplace_back(v);
        g[v].emplace_back(u);
    }

    dfs(0, -1);

    cout << max(dp[0][0], dp[0][1]) << '\n';
    get_ans(0);
}