#include <bits/stdc++.h>
using namespace std;
struct UnionFind {
	vector<int> par, rank, size;
	UnionFind(int n) : par(n), rank(n, 0), size(n, 1) {
		iota(par.begin(), par.end(), 0);
	}
	int find(int i) { return (par[i] == i ? i : (par[i] = find(par[i]))); }
	bool same(int i, int j) { return find(i) == find(j); }
	void merge(int i, int j) {
		if((i = find(i)) == (j = find(j))) return;
		if(rank[i] > rank[j]) swap(i, j);
		par[i] = j;
		size[j] += size[i];
		if(rank[i] == rank[j]) rank[j]++;
	}
};
struct E { int u, v, weight; };

	int k;
	vector<string> dna(1010);

int cnt(int i, int j)
{
	int ret = 0;
	for(int l = 0; l < k; ++l)
		ret += dna[i][l] != dna[j][l];
	return ret;
}

bool operator<(const E &l, const E &r) {
	return cnt(l.u, l.v) < cnt(r.u, r.v);
}
int main() {
	int n;
	cin >> n >> k;
	for (int i =0; i < n; i++) cin >> dna[i];
	vector<E> edges;
	edges.reserve(n*(n-1)/2);
	for(int i = 0; i < n; ++i)
		for(int j = i+1; j < n; ++j) {
			edges.push_back({i, j, 0});
		}
	sort(edges.begin(), edges.end());
	int cost = 0, count = 0;
	UnionFind uf(n);
	for(auto &e : edges) {
		if(!uf.same(e.u, e.v)) {
			cost += cnt(e.u, e.v);
			uf.merge(e.u, e.v);
			e.v = -e.v;
			if(++count == n - 1) break;
		}
	}
	cout << cost << '\n';
	for(auto e : edges)
		if(e.v < 0) cout << e.u << ' ' << -e.v << '\n';
	return 0;
}