#include<bits/stdc++.h>
using namespace std;
const long long inf = (long long) 1e18 + 10;
const int inf1 = (int) 1e9 + 10;
#define int long long
#define dbl long double
#define endl '\n'
#define sc second
#define fr first
#define mp make_pair
#define pb push_back
#define all(x) x.begin(), x.end()
const int maxn = 4e5+10;
int n, m, b[maxn], t[maxn], in[maxn], done[maxn], out[maxn];
vector<int> pos[maxn], g[maxn], gt[maxn];
int mnin(int u) {
int ans = 3;
for(auto v : g[u]) {
ans = min(ans, in[v]);
}
return ans;
}
void solve() {
cin >> n >> m;
vector<int> ept;
int cntdone = 0;
for(int i = 1; i <= m; i++) {
cin >> b[i] >> t[i];
if(b[i] == 0) {
ept.pb(i);
continue;
}
if(b[i] != 0) pos[b[i]].pb(i);
if(t[i] != 0) pos[t[i]].pb(i+m);
if(b[i] == t[i]) {
done[b[i]] = 1; cntdone++;
}
else if(b[i] != 0 && t[i] != 0) {
g[t[i]].pb(b[i]);
gt[b[i]].pb(t[i]);
out[t[i]]++;
in[b[i]]++;
}
}
set<pair<pair<int,int>,int>> ot; // out, minin, i
for(int i = 1; i <= n; i++) {
if(done[i] == 0 && in[i] == 0) {
// ot[out[i]].pb(i);
ot.insert(mp(mp(out[i],mnin(i)),i));
}
}
vector<pair<int,int>> ans;
while(true) {
if(ot.size()) {
int i = ot.begin()->sc;
ot.erase(ot.begin());
if(done[i]) continue;
if(out[i] == 0) {
// int i = ot[0].back(); ot[0].pop_back();
// if(done[i]) continue;
done[i] = 1;
cntdone++;
sort(all(pos[i]));
int p1 = pos[i][0];
int p2 = pos[i][1];
ans.pb(mp(p1,p2));
b[p1] = 0;
t[p2] = i;
ept.pb(p1);
pos[i][0] = p2+m;
}
else if(out[i] == 1) {
// int i = ot[1].back(); ot[1].pop_back();
// if(done[i]) continue;
done[i] = 1;
cntdone++;
sort(all(pos[i]));
int p1 = pos[i][0];
int p2 = pos[i][1];
ans.pb(mp(p2,p1));
t[p2-m] = 0;
for(auto x : g[i]) {
if(--in[x] == 0) {
ot.insert(mp(mp(out[x],mnin(x)),x));
}
else {
for(auto y : gt[x]) {
if(!done[y]) {
ot.insert(mp(mp(out[y],mnin(y)),y));
}
}
}
}
t[p1] = i;
pos[i][1] = p1+m;
}
else if(out[i] == 2 && ept.size()) {
// int i = ot[2].back(); ot[2].pop_back();
// if(done[i]) continue;
done[i] = 1;
cntdone++;
int e = ept.back(); ept.pop_back();
sort(all(pos[i]));
int p1 = pos[i][0];
int p2 = pos[i][1];
ans.pb(mp(p1,e));
ans.pb(mp(p2,e));
t[p1-m] = 0;
t[p2-m] = 0;
for(auto x : g[i]) {
if(--in[x] == 0) {
ot.insert(mp(mp(out[x],mnin(x)),x));
}
else {
for(auto y : gt[x]) {
if(!done[y]) {
ot.insert(mp(mp(out[y],mnin(y)),y));
}
}
}
}
b[e] = i;
t[e] = i;
pos[i][0] = e;
pos[i][1] = e+m;
}
else {
break;
}
}
else if(ept.size() && cntdone != n) {
for(int j = 1; j <= m; j++) {
if(b[j] != t[j]) {
int i = t[j];
int e = ept.back(); ept.pop_back();
sort(all(pos[i]));
ans.pb(mp(j,e));
t[j] = 0;
b[e] = i;
if(pos[i][0] == j+m) pos[i][0] = e;
else pos[i][1] = e;
if(--in[b[j]] == 0) {
ot.insert(mp(mp(out[b[j]],mnin(b[j])),b[j]));
// ot[out[b[j]]].pb(b[j]);
}
assert((in[b[j]] == 0 && out[b[j]] == 1));
assert(g[i].size() == 1);
g[i].clear();
out[i] = 0;
break;
}
}
}
else {
break;
}
}
// if(m-n != 0) {
// assert(ept.size() != 0);
// }
for(int i = 1; i <= n; i++) {
if(done[i] == 0) {
cout << -1 << endl;
return;
}
}
cout << ans.size() << endl;
for(auto x : ans) {
if(x.fr > m) x.fr-= m;
if(x.sc > m) x.sc-= m;
cout << x.fr << " " << x.sc << endl;
}
}
int32_t main() {
ios::sync_with_stdio(false); cin.tie(0);
// freopen("in.in", "r", stdin);
// freopen("out.out", "w", stdout);
int tt = 1;
// cin >> tt;
while(tt--) {
solve();
}
}
