#include <bits/stdc++.h>
using namespace std;
struct E { int u, v, weight; };
bool operator<(const E& l, const E& r){ return l.weight < r.weight; }
int main() {
	int n, k; cin >> n >> k;
	vector<string> dna(n);
	for(auto &x : dna) cin >> x;
	int w = 0;
	for(int i = 0; i < n-1; ++i)
		for(int l = 0; l < k; ++l)
			w += dna[i][l] != dna[i+1][l];
	cout << w << '\n';
	for(int i = 0; i < n-1; ++i)
		cout << i << ' ' << i+1 << '\n';
	return 0;
}