#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<ll, ll>;
int enc(array<int, 8> &a) {
int b = 0;
for (int i : a) b = 5*b + i;
return b;
}
array<int, 8> dec(int b) {
array<int, 8> a;
for (int i = 7; i >= 0; --i) {
a[i] = b % 5;
b /= 5;
}
return a;
}
void solve() {
int n, m;
cin >> n >> m;
vector<array<int, 2>> g(m+1);
vector<vector<int>> pos(n+1);
vector<int> abv(n+1, 0);
set<int> rem;
vector<int> f;
for (int i = 0; i < m; ++i) {
int a, b;
cin >> a >> b;
g[i][0] = a;
g[i][1] = b;
if (a) pos[a].push_back(i);
if (b) pos[b].push_back(i);
if (a != 0 && a != b) {
rem.insert(a);
if (b != 0) rem.insert(b);
}
if (b != 0) ++abv[a];
if (a == 0 && b == 0) f.push_back(i);
}
vector<pii> ans;
set<int> nx, nx2;
for (int i = 1; i <= n; ++i) if (rem.count(i) && abv[i] == 0) {
if (g[pos[i][0]][0] == i || g[pos[i][1]][0] == i) nx.insert(i);
else nx2.insert(i);
}
while (rem.size()) {
if (nx.size()) {
int a = *nx.begin();
int i = pos[a][0];
int j = pos[a][1];
if (g[i][0] == a && g[j][0] == a) {
ans.push_back({j, i});
swap(g[i][1], g[j][0]);
pos[a][1] = i;
f.push_back(j);
} else if (g[i][0] == a && g[j][1] == a) {
ans.push_back({j, i});
swap(g[i][1], g[j][1]);
pos[a][1] = i;
if (--abv[g[j][0]] == 0) {
if (g[pos[g[j][0]][0]][0] == g[j][0] || g[pos[g[j][0]][1]][0] == g[j][0]) nx.insert(g[j][0]);
else nx2.insert(g[j][0]);
}
} else {
ans.push_back({i, j});
swap(g[j][1], g[i][1]);
pos[a][0] = i;
if (--abv[g[i][0]] == 0) {
if (g[pos[g[i][0]][0]][0] == g[i][0] || g[pos[g[i][0]][1]][0] == g[i][0]) nx.insert(g[i][0]);
else nx2.insert(g[i][0]);
}
}
nx.erase(a);
rem.erase(a);
} else if (nx2.size()) {
int a = *nx2.begin();
int i = pos[a][0];
int j = pos[a][1];
if (f.empty()) {
cout << "-1\n";
return;
}
int k = f.back();
f.pop_back();
ans.push_back({i, k});
ans.push_back({j, k});
swap(g[k][0], g[i][1]);
swap(g[k][1], g[j][1]);
pos[a][0] = k;
pos[a][1] = k;
if (--abv[g[i][0]] == 0) {
if (g[pos[g[i][0]][0]][0] == g[i][0] || g[pos[g[i][0]][1]][0] == g[i][0]) nx.insert(g[i][0]);
else nx2.insert(g[i][0]);
}
if (--abv[g[j][0]] == 0) {
if (g[pos[g[j][0]][0]][0] == g[j][0] || g[pos[g[j][0]][1]][0] == g[j][0]) nx.insert(g[j][0]);
else nx2.insert(g[j][0]);
}
nx2.erase(a);
rem.erase(a);
} else {
int a = *rem.begin();
int i = pos[a][0];
int j = pos[a][1];
if (f.empty()) {
cout << "-1\n";
return;
}
int k = f.back();
f.pop_back();
if (g[i][1] == a) {
ans.push_back({i, k});
swap(g[k][0], g[i][1]);
pos[a][0] = k;
if (--abv[g[i][0]] == 0) {
if (g[pos[g[i][0]][0]][0] == g[i][0] || g[pos[g[i][0]][1]][0] == g[i][0]) nx.insert(g[i][0]);
else nx2.insert(g[i][0]);
}
} else {
ans.push_back({j, k});
swap(g[k][0], g[j][1]);
pos[a][1] = k;
if (--abv[g[j][0]] == 0) {
if (g[pos[g[j][0]][0]][0] == g[j][0] || g[pos[g[j][0]][1]][0] == g[j][0]) nx.insert(g[j][0]);
else nx2.insert(g[j][0]);
}
}
}
}
cout << ans.size() << '\n';
for (pii p : ans) cout << p.first+1 << ' ' << p.second+1 << '\n';
}
int main() {
cin.tie(0) -> sync_with_stdio(0);
solve();
}
