#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define int ll
#define ft first
#define sc second
#define pb push_back
#define all(x) x.begin(), x.end()
#define pi pair<int, int>
#define vi vector<int>
#define tii tuple<int, int, int>
#define tiii tuple<int, int, int, int>
#define tiiii tuple<int, int, int, int, int>
#define vpi vector<pi>
#define vtii vector<tii>
#define vtiii vector<tiii>
const int N = (1 << 20), MOD = 1e9 + 7, INF = int(1e18);
int arr[N], poz[N];
vpi ans;
bool vis[N];
int cnt, e;
int md(int x, int m){
if(x == m) return x;
if(x == 2 * m) return m;
return x % m;
}
queue<int> cur;
bool empt(int x){
return (arr[x] == 0);
}
int psh(){
while(!cur.empty() && !empt(cur.front())) cur.pop();
if(cur.empty()) return -1;
return cur.front();
}
int wsk[N], k[N];
vi z[N];
bool st[N], gd[N];
void mv(int x, int p, int m, int n){
p = md(p, m);
int v = md(poz[x], m);
if(poz[x] > m) cur.push(poz[x]);
ans.pb({v, p});
if(arr[p + m] == 0){
arr[p + m] = x;
arr[poz[x]] = 0;
poz[x] = p + m;
}else{
arr[p] = x;
arr[poz[x]] = 0;
poz[x] = p;
gd[md(x, n)] = 1;
cnt--;
}
}
bool check_clr(int c, int n, int m){
c = md(c, n);
if(gd[c]) return 1;
int a = c, b = c + n;
if(poz[a] > poz[b]) swap(a, b);
int x = poz[a], y = poz[b];
if(x > m && arr[x - m] != 0) return 0;
if(y > m && arr[y - m] != 0) return 0;
if(y <= m) return 0;
mv(a, y, m, n);
if(x <= m){
check_clr(md(arr[x + m], n), n, m);
}
return 1;
}
int dfs(int x, int u, int m, int n){
if(vis[x]){
if(k[x] == 0) k[x] = x;
return k[x];
}
vis[x] = 1;
if(u <= m){
wsk[x] = arr[u + m];
st[arr[u + m]] = 1;
k[x] = dfs(arr[u + m], u + m, m, n);
return k[x];
}
int a;
if(x > n) a = x - n;
else a = x + n;
if(poz[a] <= m){
wsk[x] = a;
st[a] = 1;
k[x] = dfs(a, poz[a], m, n);
return k[x];
}
wsk[x] = a;
st[a] = 1;
k[x] = a;
return k[x];
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL); cout.tie(NULL);
// random_device rd;
// mt19937_64 gen(rd());
int n, m;
cin >> n >> m;
cnt = n;
for(int i = 1; i <= m; i++){
cin >> arr[i + m] >> arr[i];
if(vis[arr[i]]) arr[i] += n;
if(arr[i] != 0) vis[arr[i]] = 1;
if(vis[arr[i + m]]) arr[i + m] += n;
vis[arr[i + m]] = 1;
poz[arr[i]] = i;
poz[arr[i + m]] = i + m;
vis[0] = 0;
}
for(int i = 1; i <= n; i++){
if(poz[i] % m != poz[i + n] % m) vis[i] = 0;
else{
cnt--;
gd[i] = 1;
}
}
for(int i = 1; i <= n; i++){
if(!gd[i]) check_clr(i, n, m);
}
for(int i = 1; i <= m; i++){
if(empt(i + m)) cur.push(i + m);
}
if(cnt > 0 && psh() == -1){
cout << "-1\n";
return 0;
}
gd[0] = 1;
for(int i = 1; i <= 2 * n; i++) vis[i] = 0;
for(int i = 1; i <= m; i++){
if(empt(i)) continue;
dfs(arr[i], i, m, n);
}
for(int i = 1; i <= 2 * n; i++){
if((st[i] && k[i] != i) || gd[md(i, n)] || wsk[i] == 0) continue;
if((md(i, n) == md(k[i], n)) || empt(poz[k[i]] - m)){
int x = poz[i];
e = psh();
mv(i, e, m, n);
check_clr(md(i, n), n, m);
check_clr(md(arr[x + m], n), n, m);
}else{
int x = md(k[i], n);
if(!z[x].empty() && md(z[x].front(), n) == md(i, n)){
int a = poz[i];
e = psh();
mv(i, e, m, n);
check_clr(md(i, n), n, m);
check_clr(md(arr[a + m], n), n, m);
z[x].clear();
}else{
z[md(k[i], n)].pb(i);
}
}
}
for(int i = 1; i <= n; i++){
if(z[i].empty()) continue;
if(int(z[i].size()) == 2 && !gd[md(z[i][0], n)] && !gd[md(z[i][1], n)]) continue;
if((int(z[i].size()) == 1 && gd[md(z[i][0], n)]) || (int(z[i].size()) == 2 && gd[md(z[i][0], n)] && gd[md(z[i][1], n)])){
z[i].clear();
continue;
}
int a;
if(!gd[md(z[i][0], n)] || int(z[i].size() == 1)) a = z[i][0];
else a = z[i][1];
int x = poz[a];
e = psh();
mv(a, e, m, n);
check_clr(md(a, n), n, m);
check_clr(md(arr[x + m], n), n, m);
z[i].clear();
}
e = psh();
while(!cur.empty() && cur.front() == e) cur.pop();
if(cnt > 0 && psh() == -1){
cout << "-1\n";
return 0;
}
cur.push(e);
for(int i = 1; i <= n; i++){
if(z[i].empty()) continue;
int x;
if(!gd[md(z[i][0], n)]){
x = poz[z[i][0]];
mv(z[i][0], psh(), m, n);
check_clr(md(arr[x + m], n), n, m);
check_clr(md(z[i][0], n), n, m);
}
if(!gd[md(z[i][1], n)]){
x = poz[z[i][1]];
mv(z[i][1], psh(), m, n);
check_clr(md(arr[x + m], n), n, m);
check_clr(z[i][1], n, m);
}
}
cout << int(ans.size()) << "\n";
for(auto [i, j] : ans) cout << i << ' ' << j << "\n";
return 0;
}
