#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define all(v) (v).begin(), (v).end()
#define pb push_back
#define REP(i,n) for(int i=0,_n=(n); i<_n; ++i)
#define FORE(i,a,b) for(int i=(a),_b=(b); i<=_b; ++i)
typedef pair<int,int> pii;
void process(){
int N,M;
cin>>N>>M;
vector<int> bot(M), top(M);
vector<vector<int>> pos(N, vector<int>(2,-1));
vector<vector<int>> adj(2*M);
vector<int> sz(M,0), emp;
emp.reserve(M);
// 1) Načíst bot/top, nastavit sz[i] a vyplnit emp
REP(i,M){
cin>>bot[i]>>top[i];
if(bot[i]>0) sz[i]++;
if(top[i]>0) sz[i]++;
}
REP(i,M){
if(sz[i]==0) emp.pb(i);
}
// 2) Naplnit pos[] a vnitro-hrany full spotů
REP(i,M){
if(bot[i]>0){
int c=bot[i]-1;
pos[c][ pos[c][0]!=-1 ] = i<<1;
}
if(top[i]>0){
int c=top[i]-1;
pos[c][ pos[c][0]!=-1 ] = i<<1|1;
if(bot[i]>0 && bot[i]!=top[i]){
adj[i<<1].pb(i<<1|1);
adj[i<<1|1].pb(i<<1);
}
}
}
// 3) Pomocná lambda pro prázdný spot
auto get_empty=[&](){
while(!emp.empty() && sz[emp.back()]>0) emp.pop_back();
if(emp.empty()){ cout<<-1<<"\n"; exit(0); }
return emp.back();
};
// 4) Napojit dvojice stejných barev
REP(c,N){
int u=pos[c][0], v=pos[c][1];
if(u>=0 && v>=0 && u!=(v^1)){
adj[u].pb(v);
adj[v].pb(u);
}
}
// 5) Přesuny
vector<pii> res;
auto mv=[&](int x,int y){
int sx=x>>1, sy=y>>1;
res.emplace_back(sx,sy);
// odebírám z x
sz[sx]--;
if(sz[sx]==0) emp.pb(sx);
// přidávám do y
if(sz[sy]==0){
// pokud tam bylo nula, spot už v emp, ale dle definice
// necháváme, get_empty se postará
}
sz[sy]++;
};
// 6) Rozplétání cest (kóduju přímo inline stejnou logiku)
vector<bool> used(2*M,false);
vector<vector<int>> paths, cycles;
// 6a) extract chains
REP(i,2*M){
if(!used[i] && adj[i].size()==1){
vector<int> path;
int u=i, p=-1;
while(true){
used[u]=true; path.pb(u);
int nxt=-1;
for(int v:adj[u]) if(v!=p){ nxt=v; break; }
if(nxt==-1) break;
p=u; u=nxt;
}
paths.pb(path);
}
}
// 6b) solve each chain
for(auto &path: paths){
int n = path.size();
for(int i=0;i+1<n;i+=2){
if(path[i+1]&1){
mv(path[i+1], path[i]);
} else {
// najdu nejdřív top-pár
int p=i;
for(;p+1<n;p+=2){
if((path[p]&1) && (path[p+1]&1)) break;
}
if(p+1<n){
int e = get_empty()<<1;
mv(path[p],e);
mv(path[p+1],e);
}
for(int j=p-2;j>=i;j-=2){
mv(path[j], path[j+1]);
}
i=p;
}
}
}
// 7) extract cycles
auto del_edge=[&](int x,int y){
auto it=find(all(adj[x]),y);
if(it!=adj[x].end()) adj[x].erase(it);
it=find(all(adj[y]),x);
if(it!=adj[y].end()) adj[y].erase(it);
};
REP(i,2*M){
if(!used[i] && adj[i].size()==2){
vector<int> cyc;
int u=i, p=-1;
while(!used[u]){
used[u]=true;
cyc.pb(u);
int nxt = (adj[u][0]==p?adj[u][1]:adj[u][0]);
p=u; u=nxt;
}
cyc.pb(cyc[0]);
// solve one cycle inline
int m=cyc.size(), ps=-1;
for(int j=0;j+1<m;j++){
if((cyc[j]&1)&&(cyc[j+1]&1)){ ps=j; break; }
}
if(ps>=0){
int e = get_empty()<<1;
mv(cyc[ps],e); mv(cyc[ps+1], e);
int prv = (ps?cyc[ps-1]:cyc[m-2]);
int nxt = (ps+2<m?cyc[ps+2]:cyc[1]);
del_edge(prv, cyc[ps]);
del_edge(cyc[ps], cyc[ps+1]);
del_edge(cyc[ps+1], nxt);
cyc.erase(cyc.begin()+ps, cyc.begin()+ps+2);
rotate(cyc.begin(), cyc.begin()+ps, cyc.end());
// zacyklení do cesty
int nn=cyc.size();
for(int j=0;j+1<nn;j+=2){
if(cyc[j+1]&1) mv(cyc[j+1],cyc[j]);
else{
int pe=j;
for(;pe+1<nn;pe+=2){
if((cyc[pe]&1)&&(cyc[pe+1]&1)) break;
}
if(pe+1<nn){
int ee = get_empty()<<1;
mv(cyc[pe],ee); mv(cyc[pe+1],ee);
}
for(int k=pe-2;k>=j;k-=2)
mv(cyc[k],cyc[k+1]);
j=pe;
}
}
} else {
cyc.pop_back();
if(cyc[0]&1){
rotate(cyc.begin(),cyc.begin()+1,cyc.end());
}
if(cyc[0]==(cyc[1]^1)){
rotate(cyc.begin(),cyc.begin()+1,cyc.end());
reverse(all(cyc));
}
int e = get_empty()<<1;
mv(cyc.back(), e);
int szc=cyc.size();
for(int j=1;j+2<szc;j+=2) mv(cyc[j],cyc[j-1]);
mv(cyc[szc-2], e);
}
}
}
cout<<res.size()<<"\n";
for(auto &p:res) cout<<p.fi+1<<" "<<p.se+1<<"\n";
}
int main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
process();
return 0;
}
