#include <iostream>
#include <algorithm>
#include <numeric>
#include <vector>
#include <string>

struct UnionFind {
    std::vector<int> par, sz;
    int gnum;

    explicit UnionFind(int n)
        : par(n), sz(n, 1), gnum(n) {
        std::iota(par.begin(), par.end(), 0);
    }

    int find(int v) {
        return (par[v] == v) ? v : (par[v] = find(par[v]));
    }

    void unite(int u, int v) {
        u = find(u), v = find(v);
        if (u == v) return;

        if (sz[u] < sz[v]) std::swap(u, v);
        sz[u] += sz[v];
        par[v] = u;
        --gnum;
    }

    bool same(int u, int v) { return find(u) == find(v); }
    bool ispar(int v) { return v == find(v); }
    int size(int v) { return sz[find(v)]; }
};

void solve() {
    int n, m;
    std::cin >> n >> m;

    std::vector<std::string> ss(n);
    for (auto& s : ss) std::cin >> s;

    std::vector<std::tuple<int, int, int>> es;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < i; ++j) {
            int c = 0;
            for (int k = 0; k < m; ++k) {
                if (ss[i][k] != ss[j][k]) ++c;
            }
            es.emplace_back(c, i, j);
        }
    }
    std::sort(es.begin(), es.end());

    int ans = 0;
    UnionFind uf(n);
    for (auto [c, u, v] : es) {
        if (uf.same(u, v)) continue;
        ans += c;
        uf.unite(u, v);
    }

    std::cout << ans << "\n";
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}