/* Solution to Evolution by Mees
 *
 * Uses Prim's algorithm, for run time O(n log(n)).
 *
 * Expected answer: ACCEPTED.
 *
 */
#include <iostream>
#include <algorithm>
#include <queue>

using namespace std;

int main() {
    int n, k; cin >> n >> k;
    vector<string> dna(n);
    for(int i = 0; i < n; i++)
        cin >> dna[i];
    vector<bool> intree(n, false);

    vector<vector<int>> dist(n, vector<int>(n, 0));

    int besti = -1;
    int bestj = -1;
    int best_dist = 100000;
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < n; j++) {
            if(i == j)
                continue;
            dist[i][j] = 0;
            for(int c = 0; c < k; c++) {
                if(dna[i][c] != dna[j][c])
                    dist[i][j]++;
            }
            if(dist[i][j] < best_dist) {
                besti = i;
                bestj = j;
                best_dist = dist[i][j];
            }
        }
    }

	int weight = 0;

    intree[besti] = true;
    intree[bestj] = true;
	vector<pair<int,int>> edges;
	edges.push_back({besti, bestj});
	weight += dist[besti][bestj];

    priority_queue<pair<int, pair<int, int>>> q;

    for(int i : {besti, bestj}) {
        for(int j = 0; j < n; j++) {
            if(!intree[j]) {
                q.push(make_pair(-dist[i][j], make_pair(i, j)));
            }
        }
    }

    while(!q.empty()) {
        auto top = q.top(); q.pop();
        int i = top.second.second;
        if(intree[i])
            continue;
        intree[i] = true;
		edges.push_back({top.second.first, i});
		weight += -top.first;
        for(int j = 0; j < n; j++) {
            if(!intree[j]) {
                q.push(make_pair(-dist[i][j], make_pair(i, j)));
            }
        }
    }

	cout << weight << '\n';
	for(auto e : edges)
		cout << e.first << ' ' << e.second << '\n';

    return 0;
}