#include<bits/stdc++.h>
using namespace std;

#define ll long long
#define all(v) v.begin(), v.end()
#define memo(v, x) memset(v, x, sizeof(v))
#define sz(v) (int)v.size()

void solve() {
    int n, m; cin >> n >> m;
    vector<vector<int>> adj(n);
    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }
    vector<int> comp, vis(n);
    function<void(int)> dfs = [&](int node) {
        comp.push_back(node);
        vis[node] = 1;
        for (auto &child : adj[node]) {
            if (!vis[child]) {
                dfs(child);
            }
        }
    };
    vector<vector<int>> comps;
    for (int i = 0; i < n; i++) {
        if (!vis[i]) {
            comp = vector<int>();
            dfs(i);
            comps.push_back(comp);
        }
    }
    vector<array<int, 2>> ans;
    priority_queue<array<int, 2>> pq;
    for (int i = 0; i < sz(comps); i++) {
        pq.push({sz(comps[i]), i});
    }
    while (sz(pq) > 1) {
        auto [sza, ida] = pq.top();
        pq.pop();
        auto [szb, idb] = pq.top();
        pq.pop();
        ans.push_back({comps[ida].back(), comps[idb].back()});
        comps[ida].pop_back(), comps[idb].pop_back();
        sza--, szb--;
        if (sza) pq.push({sza, ida});
        if (szb) pq.push({szb, idb});
    }
    cout << sz(ans) << '\n';
    for (auto &[u, v] : ans) {
        cout << u << " " << v << '\n';
    } 
}

int32_t main() {
    cin.tie(0) -> sync_with_stdio(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    int tc = 1;
    // cin >> tc;
    for (int t = 1; t <= tc; t++) {
        solve();
    }
    return 0;
}