#include <bits/stdc++.h>
using namespace std;
int n, m;
vector <int> a, b;
int main() {
cin >> n >> m;
a = b = vector <int> (m+1);
for (int i = 1; i <= m; i++)
cin >> a[i] >> b[i];
int ans = 0;
vector <pair <int, int>> steps;
while (true) {
set <pair <int, int>> to_add;
for (int i = 1; i <= m; i++) {
if (b[i] == 0 && a[i] != 0)
to_add.insert({a[i], i});
}
bool done = false;
for (int i = 1; i <= m; i++) {
if (b[i] == a[i] || b[i] == 0)
continue;
if (a[i] == b[i] && a[i] != 0)
continue;
auto it = to_add.lower_bound({b[i], 0});
if (it != to_add.end() && it->first == b[i]) {
done = true;
ans ++;
steps.push_back({i, it->second});
b[it->second] = b[i];
b[i] = 0;
break;
}
}
if (done)
continue;
to_add.clear();
for (int i = 1; i <= m; i++) {
if (b[i] != 0)
continue;
if (a[i] == b[i] && a[i] != 0)
continue;
auto it = to_add.lower_bound({a[i], 0});
if (it != to_add.end() && it->first == a[i]) {
ans++;
steps.push_back({i, it->second});
b[it->second] = a[i];
a[i] = 0;
done = true;
break;
} else if (a[i] != 0)
to_add.insert({a[i], i});
}
if (done)
continue;
set <int> seen;
int empty = -1;
for (int i = 1; i <= m; i++) {
if (a[i] == 0)
empty = i;
else
seen.insert(a[i]);
}
for (int i = 1; i <= m; i++) {
if (b[i] == 0)
continue;
if (a[i] == b[i] && a[i] != 0)
continue;
if (seen.find(b[i]) == seen.end()) {
if (empty == -1) {
cout << -1 << '\n';
return 0;
}
a[empty] = b[i];
b[i] = 0;
done = true;
ans++;
steps.push_back({i, empty});
break;
}
}
if (done)
continue;
if (!done)
break;
}
cout << ans << '\n';
for (auto i : steps)
cout << i.first << ' ' << i.second << '\n';
}
