#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define in insert
#define all(x) x.begin(),x.end()
#define pb push_back
#define eb emplace_back
#define ff first
#define ss second
// #define int long long
typedef long long ll;
typedef vector<int> vi;
typedef set<int> si;
typedef multiset<int> msi;
typedef pair<int, int> pii;
typedef vector<pii> vpii;
template<typename T, typename U>
ostream & operator << (ostream &out, const pair<T, U> &c) {
out << c.first << ' ' << c.second;
return out;
}
template<typename T>
ostream & operator << (ostream &out, vector<T> &v) {
const int sz = v.size();
for (int i = 0; i < sz; i++) {
if (i) out << ' ';
out << v[i];
}
return out;
}
template<typename T>
istream & operator >> (istream &in, vector<T> &v) {
for (T &x : v) in >> x;
return in;
}
template<typename T>
void mxx(T &a, T b){if(b > a) a = b;}
template<typename T>
void mnn(T &a, T b){if(b < a) a = b;}
const int mxn = 2e5 + 10;
int v[mxn][2], done[mxn];
vector<pair<int, int>> g[mxn];
void go() {
int n, m;
cin >> n >> m;
set<int> em;
for(int i = 1; i <= m; i++) {
for(int j = 0; j < 2; j++) {
cin >> v[i][j];
if(v[i][j]) g[v[i][j]].eb(j, i);
}
if(v[i][0] == 0) em.in(i);
else if(v[i][0] == v[i][1]) done[v[i][0]] = 1;
}
vector<pair<int, int>> ans;
for(int i = 1; i <= n; i++) {
if(done[i]) continue;
sort(all(g[i]));
if(g[i][0].ff == 1) {
if(em.empty()) {
cout << -1 << '\n';
return;
}
int x = *em.begin();
ans.eb(g[i][0].ss, x);
ans.eb(g[i][1].ss, x);
v[g[i][0].ss][1] = v[g[i][1].ss][1] = 0;
em.erase(x);
v[x][0] = v[x][1] = i;
done[i] = 1;
}
}
function<void(int)> dfs = [&](int x) -> void {
if(done[x]) return;
if(g[x][1].ff == 0 && v[g[x][0].ss][1] == 0 && v[g[x][1].ss][1] == 0) {
em.in(g[x][0].ss);
ans.eb(g[x][0].ss, g[x][1].ss);
v[g[x][0].ss][0] = 0;
done[x] = 1;
} else if(g[x][1].ff == 1 && v[g[x][0].ss][1] == 0) {
ans.eb(g[x][1].ss, g[x][0].ss);
v[g[x][1].ss][1] = 0;
done[x] = 1;
dfs(v[g[x][1].ss][0]);
}
};
for(int i = 1; i <= n; i++) {
if(done[i]) continue;
dfs(i);
}
for(int i = 1; i <= n; i++) {
if(done[i]) continue;
assert(g[i][1].ff == 1);
int x = g[i][0].ss, y = g[i][1].ss;
if(em.empty()) {
cout << -1 << '\n';
return;
}
int p = *em.begin();
ans.eb(y, p);
v[y][1] = 0;
v[p][0] = i;
g[i][1] = {0, p};
dfs(v[y][0]);
}
for(int i = 1; i <= n; i++) {
if(!done[i]) {
cout << -1 << '\n';
return;
}
}
cout << ans.size() << '\n';
for(auto it : ans) {
cout << it << '\n';
}
}
signed main() {
// #ifndef ONO
// freopen("file.in", "r", stdin);
// freopen("file.out", "w", stdout);
// #endif
fast;
int t = 1;
// cin >> t;
while(t--) {
go();
}
return 0;
}
