#include <bits/stdc++.h>

using namespace std;

#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif

#define ll long long
#define ld long double
#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
#define FOR(i,l,r) for(int i=(l);i<=(r);i++)
#define ROF(i,r,l) for(int i=(r);i>=(l);i--)
int inf=1000000007;
ll infl=1000000000000000007;
ll mod=1000000007;
ll mod1=998244353;

const int N=407;

vector<int>V[N][N];
int x[N*N];

vector<pair<int,int>>G[2*N];
vector<int>ord;

void Euler(int v,int k)
{
	while(sz(G[v])>0)
	{
		int u=G[v].back().st,t=G[v].back().nd;
		G[v].pop_back();
		Euler(u,t);
	}
	if(k!=-1) ord.pb(k);
}

signed main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	int n,m;
	cin>>n>>m;
	FOR(i,0,n*m-1)
	{
		cin>>x[i];
		x[i]--;
		V[i/m][x[i]].pb(i);
	}
	FOR(i,0,n-1)
	{
		FOR(j,0,m-1)
		{
			if(sz(V[i][j])==0) G[j+n].pb({i,-1});
			FOR(l,1,sz(V[i][j])-1) G[i].pb({j+n,V[i][j][l]});
		}
	}
	vector<pair<int,int>>ans;
	FOR(i,0,n-1) 
	{
		if(sz(G[i])==0) continue;
		ord.clear();
		Euler(i,-1);
		ans.pb({ord[0],n*m});
		FOR(j,1,sz(ord)-1) ans.pb({ord[j],ord[j-1]});
		ans.pb({n*m,ord.back()});
	}
	cout<<sz(ans)<<endl;
	for(auto [a,b]:ans) cout<<a+1<<" "<<b+1<<endl;
	
	
	
	return 0;
}