#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> P;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<vvvi> vvvvi;
typedef vector<P> vp;
typedef vector<vp> vvp;
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define REP(i,k,n) for(ll i=(ll)(k);i<(ll)(n);i++)
#define pb emplace_back
#define fi first
#define se second
#define all(a) a.begin(),a.end()
template<class T> bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<class T> bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T> void out(T a){cout<<a<<'\n';}
const ll inf=1001001001001001001;

int main(){
    ll n,m;cin>>n>>m;
    vvi v(n,vi(m));
    rep(i,n)rep(j,m){
        cin>>v[i][j];v[i][j]--;
    }
    vvvi al(n,vvi(m));
    rep(i,n)rep(j,m)al[i][v[i][j]].pb(i*m+j);
    vvp g(n+m);
    rep(i,n)rep(j,m){
        if(al[i][j].size()==0){
            g[i].pb(j+n,-1);
        }
        else{
            while(al[i][j].size()>1){
                g[j+n].pb(i,al[i][j].back());
                al[i][j].pop_back();
            }
        }
    }
    vi edge;
    auto dfs=[&](auto && self, ll i) -> void {
        while(g[i].size()){
            P x=g[i].back();g[i].pop_back();
            self(self,x.fi);
            if(x.se!=-1)edge.pb(x.se);
        }
    };
    vp ans;
    rep(j,m)if(g[j+n].size()){
        edge=vi(0);
        dfs(dfs,j+n);
        reverse(all(edge));
        ll cur=n*m;
        for(ll x:edge){
            ans.pb(x,cur);
            cur=x;
        }
        ans.pb(n*m,cur);
    }
    out(ans.size());
    for(auto x:ans)cout<<x.fi+1<<' '<<x.se+1<<endl;
}