330B - Road Construction - CodeForces Solution

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

typedef long long ll;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    
    ll n, m;
    cin>>n>>m;
    
    set<ll> st;
    
    for(ll i=1 ; i<=n ; i++)
    st.insert(i);
    
    for(ll i=0 ; i<m ; i++){
        ll a, b;
        cin>>a>>b;
        
        if(st.count(a));
        st.erase(a);
        
        if(st.count(b))
        st.erase(b);
    }
    
    ll center = *(st.begin());
        
    cout<<n-1<<"\n";
    
    for(ll i=1 ; i<=n ; i++){
        if(i == center)
        continue;
            
        cout<<center<<" "<<i<<"\n";
    }
    
	return 0;
}