#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;
vi elms[200001];
vi poz[200001];
vi graph[200001];
bool odw[200001];
int up_cnt[200001];
int down_cnt[200001];
int n,m;
vector<vi> comps;
vi comp;
vector<pii> opers;
unordered_set<int> zero_ind;
void add_oper(int a, int b)
{
elms[b].pb(elms[a].back());
elms[a].pop_back();
if(siz(elms[a]) == 0) zero_ind.insert(a);
if(siz(elms[b]) == 1) zero_ind.erase(b);
opers.pb({a+1,b+1});
}
void dfs(int v)
{
odw[v] = 1;
comp.pb(v);
forall(it,graph[v]) if(!odw[it]) dfs(it);
}
pii get_val(vi c)
{
int up_c = 0;
bool is_path = 0;
forall(it,c)
{
if(siz(elms[it]) == 1) is_path = 1;
if(siz(elms[it]) == 2 && up_cnt[elms[it][1]] == 2) up_c++;
}
up_c /= 2;
if(!is_path)
{
if(up_c == 0) return {1,-1};
return {up_c >= 2 ? 2 : 1,0};
}
return {up_c >= 1 ? 1 : 0,1};
}
void solve(vi c)
{
bool is_path = 0;
forall(it,c) if(siz(elms[it]) == 1) is_path = 1;
if(!is_path)
{
int f = *zero_ind.begin();
int d = -1;
forall(it,c) if(up_cnt[elms[it][1]] == 2) d = elms[it][1];
if(d != -1)
{
forall(it,poz[d]) add_oper(it,f);
comp = {};
forall(it,c) odw[it] = 0;
odw[poz[d][0]] = 1;
dfs(poz[d][1]);
c = comp;
c.pb(poz[d][0]);
}
else
{
if(elms[c[0]][1] != elms[c[1]][0]) reverse(all(c));
add_oper(c[0],f);
int pop = c[0];
for(int i = siz(c)-1; i >= 1; i--)
{
add_oper(c[i],pop);
pop = c[i];
}
add_oper(c[1],f);
return;
}
}
int ind = 0;
while(ind < siz(c)-1)
{
if(elms[c[ind]][0] == elms[c[ind+1]][1])
{
add_oper(c[ind+1],c[ind]);
ind++;
continue;
}
int p = -1;
for(int i = ind+1; i < siz(c)-1; i++)
{
if(up_cnt[elms[c[i]][1]] == 2)
{
p = i;
break;
}
}
if(p != -1)
{
int f = *zero_ind.begin();
add_oper(c[p],f);
add_oper(c[p+1],f);
for(int i = p-1; i >= ind; i--)
{
add_oper(c[i],c[i+1]);
}
ind = p+1;
}
else
{
for(int i = siz(c)-2; i >= ind; i--)
{
add_oper(c[i],c[i+1]);
}
return;
}
}
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
cin >> m >> n;
int zero_cnt = 0;
rep(i,n)
{
int a,b;
cin >> a >> b;
up_cnt[b]++;
down_cnt[a]++;
if(a == 0)
{
zero_cnt++;
zero_ind.insert(i);
}
if(a != 0) elms[i].pb(a);
if(b != 0) elms[i].pb(b);
}
rep(i,n) forall(it,elms[i]) if(it != 0) poz[it].pb(i);
rep2(i,1,m)
{
graph[poz[i][0]].pb(poz[i][1]);
graph[poz[i][1]].pb(poz[i][0]);
}
rep(i,n)
{
if(odw[i]) continue;
if(siz(graph[i]) == 1)
{
comp = {};
dfs(i);
comps.pb(comp);
}
}
int ans = m;
rep2(i,1,m) if(up_cnt[i] == 2) ans++;
rep(i,n)
{
if(odw[i]) continue;
comp = {};
dfs(i);
if(siz(comp) != 1) comps.pb(comp);
else if(siz(elms[i]) == 2) ans--;
}
vector<pair<pii,int>> vs;
rep(i,siz(comps))
{
vs.pb({get_val(comps[i]),i});
}
sort(all(vs));
forall(it,vs)
{
if(zero_cnt < it.ff.ff)
{
cout << "-1\n";
return 0;
}
solve(comps[it.ss]);
if(it.ff.ss == -1) ans++;
else zero_cnt += it.ff.ss;
}
cout << ans << '\n';
forall(it,opers) cout << it.ff << " " << it.ss << "\n";
}
