import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const L = readInt();
      auto G = new string[N];
      foreach (u; 0 .. N) {
        G[u] = readToken();
      }
      
      auto dist = new int[][](N, N);
      foreach (u; 0 .. N) foreach (v; u + 1 .. N) {
        int cnt;
        foreach (l; 0 .. L) {
          if (G[u][l] != G[v][l]) {
            ++cnt;
          }
        }
        dist[u][v] = cnt;
      }
      
      auto used = new bool[N];
      auto ds = new int[N];
      ds[] = L + 1;
      ds[0] = 0;
      
      int ans;
      foreach (h; 0 .. N) {
        int um = -1;
        foreach (u; 0 .. N) {
          if (!used[u]) {
            if (um == -1 || ds[um] > ds[u]) {
              um = u;
            }
          }
        }
        ans += ds[um];
        used[um] = true;
        foreach (v; 0 .. N) {
          if (!used[v]) {
            chmin(ds[v], dist[um][v]);
          }
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}