#include <bits/stdc++.h>
using namespace std;
using LL = long long int;
#define incID(i, l, r) for(int i = (l)    ; i <  (r); i++)
#define decID(i, l, r) for(int i = (r) - 1; i >= (l); i--)
#define incII(i, l, r) for(int i = (l)    ; i <= (r); i++)
#define decII(i, l, r) for(int i = (r)    ; i >= (l); i--)
#define inc(i, n)  incID(i, 0, n)
#define dec(i, n)  decID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec1(i, n) decII(i, 1, n)
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))
#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define FI first
#define SE second
#define FR front()
#define BA back()
#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
auto setmin   = [](auto & a, auto b) { return (b <  a ? a = b, true : false); };
auto setmax   = [](auto & a, auto b) { return (b >  a ? a = b, true : false); };
auto setmineq = [](auto & a, auto b) { return (b <= a ? a = b, true : false); };
auto setmaxeq = [](auto & a, auto b) { return (b >= a ? a = b, true : false); };
#define SI(v) static_cast<int>(v.size())
#define RF(e, v) for(auto & e: v)
#define until(e) while(! (e))
#define if_not(e) if(! (e))
#define ef else if
#define UR assert(false)
// 以下、入出力関係のテンプレ案のテスト
auto * IS = & cin;
// input elements (as a tuple)
template<typename U, int I> void in_(U & t) { }
template<typename U, int I, typename A, typename ... B> void in_(U & t) { (* IS) >> get<I>(t); in_<U, I + 1, B ...>(t); }
template<typename ... T> auto in() { tuple<T ...> t; in_<tuple<T ...>, 0, T ...>(t); return t; }
// input a array
template<typename T, int N> auto ain() { array<T, N> a; inc(i, N) { (* IS) >> a[i]; } return a; }
// input a (multi-dimensional) vector
template<typename T> T vin() { return * istream_iterator<T>(* IS); }
template<typename T, typename N, typename ... M> auto vin(N n, M ... m) {
	vector<decltype(vin<T, M ...>(m ...))> v(n); inc(i, n) { v[i] = vin<T, M ...>(m ...); } return v;
}
// input multi-column (as a tuple of vector)
template<typename U, int I> void colin_(U & t) { }
template<typename U, int I, typename A, typename ... B> void colin_(U & t) {
	get<I>(t).emplace_back(* istream_iterator<A>(* IS)); colin_<U, I + 1, B ...>(t);
}
template<typename ... T> auto colin(int n) {
	tuple<vector<T> ...> t; inc(i, n) { colin_<tuple<vector<T> ...>, 0, T ...>(t); } return t;
}
auto * OS = & cout;
string SP = " ", LN = "\n";
// output elements
void out() { (* OS) << LN << flush; }
template<typename A                > void out(A && a            ) { (* OS) << a      ; out(     ); }
template<typename A, typename ... B> void out(A && a, B && ... b) { (* OS) << a << SP; out(b ...); }
// output a (multi-dimensional) vector
template<typename T> ostream & operator<<(ostream & os, vector<T> const & v) {
	inc(i, v.size()) { os << (i == 0 ? "" : SP) << v[i]; } return os << flush;
}
template<typename T> void vout_(T && v) { (* OS) << v; }
template<typename T, typename A, typename ... B> void vout_(T && v, A a, B ... b) {
	for(auto && w: v) { vout_(w, b ...); (* OS) << a; }
}
template<typename T, typename ... A> void vout(T && v, A ... a) { vout_(v, a ...); out(); }

// ---- ----

class UnionFind {
private:
	int n, s;
	vector<int> t;
	vector<vector<int>> v;
public:
	UnionFind(int arg_n = 0) { init(arg_n); }
	void init(int arg_n) {
		n = s = arg_n;
		t.clear();
		v.clear();
		inc(i, n) { t.PB(i); v.EB(1, i); }
	}
	int get_n() { return n; }
	int size() { return s; }
	int id(int x) { return t.at(x); }
	const vector<vector<int>> & get_v() { return v; }
	bool unite(int x, int y) {
		x = id(x);
		y = id(y);
		if(x == y) { return false; }
		if(v[x].size() < v[y].size()) { swap(x, y); }
		for(auto & e: v[y]) { v[x].PB(e); t[e] = x; }
		v[y].clear();
		s--;
		return true;
	}
	bool same(int x, int y) { return (id(x) == id(y)); }
	const vector<int> & operator[](int x) { return v[id(x)]; }
	friend ostream & operator<<(ostream & os, const UnionFind & uf) {
		inc(i, uf.n) { os << i << ": "; for(auto & e: uf.v[i]) { os << e << " "; } os << "\n"; }
		return os;
	}
};

// ---- ----

int main() {
	auto [n, l] = in<int, int>();
	auto s = vin<string>(n);
	
	vector<array<int, 3>> v;
	inc(i, n) {
	inc(j, i) {
		int c = 0;
		inc(k, l) { if(s[i][k] != s[j][k]) { c++; } }
		v.PB({ c, i, j });
	}
	}
	sort(ALL(v));
	
	UnionFind uf(n);
	int ans = 0;
	for(auto [c, a, b]: v) {
		if(uf.unite(a, b)) { ans += c; }
	}
	out(ans);
}