#include <bits/stdc++.h>
using namespace std;
#define ll long long
const ll mmod = 998244353;
#define vl vector<ll>
#define vvl vector<vl>
#define pl pair<ll, ll>
#define vb vector<bool>
ll n, m;
vvl stk;
vl emp;
vvl at;
vector<pl> res;
void fail() {
cout << -1 << '\n';
exit(0);
}
void atob(ll a, ll b) {
ll v = stk[a].back();
stk[a].pop_back();
stk[b].push_back(v);
for (ll &x : at[v]) if (x == a) { x = b; break; }
res.emplace_back(a, b);
}
ll other(ll x, ll p) {
// at[x] has size 2
return at[x][0] ^ at[x][1] ^ p;
}
ll shift(ll p) {
ll v = stk[p][0];
ll o = other(v, p);
while (stk[o].size() == 2 && stk[o][1] == v) {
atob(o, p);
p = o;
v = stk[p][0];
o = other(v, p);
}
return p;
}
ll eqtop(ll p) {
ll v = stk[p][1];
ll o = other(v, p);
while (stk[o][0] == v) {
p = o;
v = stk[p][1];
o = other(v, p);
}
return p;
}
void take_care(ll p) {
if (emp.empty()) fail();
if (stk[p].empty()) {
emp.push_back(p);
return;
}
// size == 1
ll v = stk[p][0];
ll e = emp.back();
ll o = other(v, p);
if (stk[o].size() == 1 || stk[o][1] == v) {
atob(o, p);
take_care(o);
return;
}
ll g = eqtop(o);
atob(g, e);
shift(g);
atob(o, p);
emp.back() = o;
take_care(e);
}
void no_more_care(ll p) {
if (stk[p].empty()) {
emp.push_back(p);
return;
}
// size == 1
ll v = stk[p][0];
ll o = other(v, p);
if (stk[o].back() == v) {
atob(o, p);
no_more_care(o);
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> m;
stk.assign(m, vl());
at.assign(n + 1, vl());
for (ll i = 0; i < m; i++) {
ll b, t;
cin >> b >> t;
for (ll j : {b, t}) if (j) {
stk[i].push_back(j);
at[j].push_back(i);
}
if (b == 0) emp.push_back(i);
}
for (ll i = 0; i < m; i++) if (stk[i].size() == 1) no_more_care(i);
for (ll i = 0; i < m; i++) if (stk[i].size() == 1) take_care(i);
for (ll i = 1; i <= n; i++) {
ll a = at[i][0], b = at[i][1];
if (a == b) continue;
if (stk[a].size() + stk[b].size() == 4 && stk[a].back() == i && stk[b].back() == i) {
if (emp.empty()) fail();
ll e = emp.back(); emp.pop_back();
atob(a, e);
atob(b, e);
a = shift(a); b = shift(b);
if (stk[a][0] == stk[b][0]) {
atob(a, b);
emp.push_back(a);
} else {
take_care(a);
}
}
}
for (ll i = 1; i <= n; i++) {
ll a = at[i][0], b = at[i][1];
if (a == b) continue;
if (emp.empty()) fail();
if (stk[a][0] != i) swap(a, b);
ll e = emp.back();
atob(a, e);
ll g = shift(a);
atob(e, g);
}
for (ll i = 1; i <= n; i++) assert(at[i][0] == at[i][1]);
cout << res.size() << '\n';
for (auto &pr : res) {
cout << pr.first + 1 << ' ' << pr.second + 1 << '\n';
}
return 0;
}
