CFR 330 B. Road Construction ( Graph Adhoc )

Problem - 330B - Codeforces
There are n vertices and at most n / 2 - 1 edges. Easy to see that there will be at least one vertex with 0 degree.

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e3 + 3;
const int MAXM = MAXN / 2;

int n, m;
int vis[ MAXN ];

void print( int x ){
    printf("%d\n", n - 1);
    for( int i = 1; i <= n; ++i )
        if( i != x )
            printf("%d %d\n", i, x);
}

int main(){
    scanf("%d%d", &n, &m);
    for( int i = 0; i < m; ++i ){
        int u, v; scanf("%d%d", &u, &v);
        vis[ u ] = vis[ v ] = 1;
    }
    for( int i = 1; i <= n; ++i )
        if( !vis[ i ] ){
            print( i );
            return 0;
        }
    assert( false );
    return 0;
}