#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;
REP(i,M){
cin>>bot[i]>>top[i];
if(bot[i]){
sz[i]++;
pos[bot[i]-1][ pos[bot[i]-1][0]!=-1 ] = i<<1;
}
if(top[i]){
sz[i]++;
pos[top[i]-1][ pos[top[i]-1][0]!=-1 ] = i<<1|1;
if(bot[i] && bot[i]!=top[i]){
adj[i<<1].pb(i<<1|1);
adj[i<<1|1].pb(i<<1);
}
}
if(bot[i]==0 && top[i]==0) emp.pb(i);
}
// helper to get an empty spot
auto get_empty=[&]{
while(!emp.empty() && sz[emp.back()]>0) emp.pop_back();
if(emp.empty()){ cout<<-1; exit(0); }
return emp.back();
};
// connect color pairs
REP(c,N) if(pos[c][0]!=(pos[c][1]^1)){
int u=pos[c][0], v=pos[c][1];
if(u>=0&&v>=0){ adj[u].pb(v); adj[v].pb(u);}
}
vector<pii> res;
auto mv=[&](int x,int y){
int ux=x>>1, uy=y>>1;
res.pb({ux,uy});
if(--sz[ux]==0) emp.pb(ux);
sz[uy]++;
};
// solve path
auto solve_path=[&](vector<int>& path){
int n=path.size();
for(int i=0;i<n;i+=2){
if(path[i+1]&1) mv(path[i+1],path[i]);
else{
int p=n;
for(int j=i;j<n;j+=2) if((path[j]&1)&&(path[j+1]&1)){ p=j; break; }
if(p<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;
}
}
};
vector<bool> used(2*M,false);
vector<vector<int>> paths, cycles;
vector<int> cnt_pt;
// 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]=1; 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;
}
int cnt=0;
for(int j=0;j+1<path.size();j+=2) if((path[j]&1)&&(path[j+1]&1)) cnt++;
paths.pb(path); cnt_pt.pb(cnt);
}
vector<int> order(paths.size()); iota(all(order),0);
sort(all(order),[&](int a,int b){return cnt_pt[a]<cnt_pt[b];});
for(int i:order) solve_path(paths[i]);
// delete edge helper
auto del_edge=[&](int x,int y){
adj[x].erase(find(all(adj[x]),y));
adj[y].erase(find(all(adj[y]),x));
};
// extract cycles
REP(i,2*M) if(!used[i] && adj[i].size()==2){
vector<int> cyc;
int u=i, p=-1;
while(!used[u]){ used[u]=1; cyc.pb(u);
int nxt= (adj[u][0]==p?adj[u][1]:adj[u][0]);
p=u; u=nxt;
}
cyc.pb(cyc[0]);
cycles.pb(cyc);
}
// solve cycles
for(auto &cycle:cycles){
int m=cycle.size(); int p=-1;
for(int i=0;i+1<m;i++) if((cycle[i]&1)&&(cycle[i+1]&1)){ p=i; break; }
if(p!=-1){
int e=get_empty()<<1;
mv(cycle[p],e); mv(cycle[p+1],e);
int prev = (p?cycle[p-1]:cycle[m-2]);
del_edge(prev,cycle[p]); del_edge(cycle[p],cycle[p+1]);
int next = (p+2<m?cycle[p+2]:cycle[1]);
del_edge(cycle[p+1],next);
cycle.erase(cycle.begin()+p, cycle.begin()+p+2);
rotate(cycle.begin(), cycle.begin()+p, cycle.end());
solve_path(cycle);
} else {
cycle.pop_back();
if(cycle[0]&1) rotate(cycle.begin(),cycle.begin()+1,cycle.end());
if(cycle[0]==(cycle[1]^1)){
rotate(cycle.begin(),cycle.begin()+1,cycle.end()); reverse(all(cycle));
}
int e=get_empty()<<1;
mv(cycle.back(),e);
for(int i=1;i+2<cycle.size();i+=2) mv(cycle[i],cycle[i-1]);
mv(cycle[cycle.size()-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;
}
