#include <bits/stdc++.h>
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define pb push_back
#define FIO ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
using namespace std;
typedef long long ll;
typedef pair <int, int> pii;
const int N = 2e5 + 7;
int n, m, nn;
int c[N], d[N], asi[N], bio[2*N];
int a[N][2];
queue <int> q, q2, fr;
vector <pii> res;
void move(int x, int y) {
res.pb({x, y});
if (!fr.empty() && fr.front() == y) fr.pop();
if (asi[x] == 1) fr.push(x);
else {
c[a[x][0]] = 1;
if (c[a[x][0]^1]) q.push(a[x][0]);
q2.push(a[x][0]);
}
if (asi[y] == 1) nn--, bio[a[y][0]] = bio[a[y][0]^1] = 1;
a[y][asi[y]++] = a[x][--asi[x]];
}
void kick(int x) {
int y = a[d[x^1]][1];
if (asi[d[y]]+asi[d[y^1]] == 4) {
int frf = fr.front();
move(d[y], frf);
move(d[y^1], frf);
}
else kick(y);
}
int main () {
FIO;
cin >> n >> m;
nn = n;
for (int i = 0; i < m; i++) {
int x, y; cin >> x >> y;
if (x == y && x) {
nn--;
continue;
}
x--; y--;
if (x == -1) fr.push(i);
if (x >= 0) x = 2*x+(bio[x]++), a[i][asi[i]++] = x, c[x] = (y == -1), d[x] = i;
if (y >= 0) y = 2*y+(bio[y]++), a[i][asi[i]++] = y, c[y] = 1, d[y] = i;
}
memset(bio, 0, sizeof bio);
for (int i = 0; i < n; i++) {
if (c[2*i] && c[2*i+1]) {
if (asi[d[2*i]]+asi[d[2*i+1]] < 4) q.push(asi[d[2*i]] == 1 ? 2*i : 2*i+1);
}
}
int ii = 0;
while (nn) {
int x = -1;
if (!q.empty()) x = q.front(), q.pop();
if (x != -1) {
if (bio[x]) continue;
bio[x] = bio[x^1] = 1;
move(d[x^1], d[x]);
continue;
}
if (fr.empty()) {
cout << "-1\n";
return 0;
}
if (!q2.empty()) {
x = q2.front(); q2.pop();
if (bio[x]) continue;
kick(x);
}
else {
while (ii < n && !(asi[ii] == 2 && !bio[a[ii][1]])) ii++;
move(a[ii][1], fr.front());
}
}
cout << res.size() << "\n";
for (auto it : res) cout << it.F+1 << " " << it.S+1 << "\n";
return 0;
}
