#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <functional>
#include <set>
#include <ctime>
#include <random>
#include <chrono>
#include <cassert>
#include <bitset>
#include <tuple>
using namespace std;

namespace {
  using Integer = long long; //__int128;
  template<class T> istream& operator >> (istream& is, vector<T>& vec){for(T& val: vec) is >> val; return is;}
  template<class T> istream& operator ,  (istream& is, T& val){ return is >> val;}
  template<class T> ostream& operator << (ostream& os, const vector<T>& vec){for(int i=0; i<vec.size(); i++) os << vec[i] << (i==vec.size()-1?"":" "); return os;}
  template<class T> ostream& operator ,  (ostream& os, const T& val){ return os << " " << val;}

  template<class H> void print(const H& head){ cout << head; }
  template<class H, class ... T> void print(const H& head, const T& ... tail){ cout << head << " "; print(tail...); }
  template<class ... T> void println(const T& ... values){ print(values...); cout << endl; }

  template<class H> void eprint(const H& head){ cerr << head; }
  template<class H, class ... T> void eprint(const H& head, const T& ... tail){ cerr << head << " "; eprint(tail...); }
  template<class ... T> void eprintln(const T& ... values){ eprint(values...); cerr << endl; }

  class range{
    long long start_, end_, step_;
   public:
    struct range_iterator{
      long long val, step_;
      range_iterator(long long v, long long step) : val(v), step_(step) {}
      long long operator * (){return val;}
      void operator ++ (){val += step_;}
      bool operator != (range_iterator& x){return step_ > 0 ? val < x.val : val > x.val;}
    };
    range(long long len) : start_(0), end_(len), step_(1) {}
    range(long long start, long long end) : start_(start), end_(end), step_(1) {}
    range(long long start, long long end, long long step) : start_(start), end_(end), step_(step) {}
    range_iterator begin(){ return range_iterator(start_, step_); }
    range_iterator   end(){ return range_iterator(  end_, step_); }
  };

  string operator "" _s (const char* str, size_t size){ return move(string(str)); }
  constexpr Integer my_pow(Integer x, Integer k, Integer z=1){return k==0 ? z : k==1 ? z*x : (k&1) ? my_pow(x*x,k>>1,z*x) : my_pow(x*x,k>>1,z);}
  constexpr Integer my_pow_mod(Integer x, Integer k, Integer M, Integer z=1){return k==0 ? z%M : k==1 ? z*x%M : (k&1) ? my_pow_mod(x*x%M,k>>1,M,z*x%M) : my_pow_mod(x*x%M,k>>1,M,z);}
  constexpr unsigned long long operator "" _ten (unsigned long long value){ return my_pow(10,value); }

  inline int k_bit(Integer x, int k){return (x>>k)&1;} //0-indexed

  mt19937 mt(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count());

  template<class T> string join(const vector<T>& v, const string& sep){
    stringstream ss; for(int i=0; i<v.size(); i++){ if(i>0) ss << sep; ss << v[i]; } return ss.str();
  }

  string operator * (string s, int k){ string ret; while(k){ if(k&1) ret += s; s += s; k >>= 1; } return ret; }

}


constexpr long long mod = 9_ten + 7;

using bits = bitset<26*300>;
using bits_small = bitset<300>;
//using bits = bitset<26*3>;

int main(){
  int h,w;
  cin >> h,w;

  vector<string> c(h);
  cin >> c;

  vector<bits> col(w);
  for(auto y : range(h) ){
    for(auto x : range(w) ){
      col[x].set( (h-y-1)*26 + c[y][x]-'a', 1 );
    }
  }

  // for(auto i : range(w) ){
  //   eprintln(col[i]);
  // }


  auto get_cost_now = [&](int i, vector<int>& s){
    int cost_now = 0;
    if(i-1 >= 0){
      if(s[i-1] != h && s[i] != h)
      cost_now += ((col[i-1]>>(s[i-1]*26)) & (col[i]>>((s[i])*26))).count();
    }
    if(i+1 < w){
      if(s[i+1] != h && s[i] != h)
      cost_now += ((col[i+1]>>(s[i+1]*26)) & (col[i]>>((s[i])*26))).count();
    }
    return cost_now;
  };

  int ans = 0;
  for(auto i : range(w-1) ){
    vector<vector<int>> dp(h+1, vector<int>(h+1, 1e8));
    dp[0][0] = 0;

    vector<bits_small> pre_l(h+1);
    for(auto h1 : range(h+1) ){
      auto tmp = ((col[i+1]>>(h1*26)) & col[i]);
      for(auto hh : range(h) ){
        pre_l[h1].set( hh, tmp[ hh*26 + c[h-1-hh][i]-'a' ] );
      }
    }
    vector<bits_small> pre_r(h+1);
    for(auto h1 : range(h+1) ){
      auto tmp = ((col[i]>>(h1*26)) & col[i+1]);
      for(auto hh : range(h) ){
        pre_r[h1].set( hh, tmp[ 26*hh + c[h-1-hh][i+1]-'a' ] );
      }
    }


    for(auto h1 : range(h+1) ){
      for(auto h2 : range(h+1) ){
        int ccc = h1 < h2 ? (pre_l[h2-h1] >> h1).count() : (pre_r[h1-h2] >> h2).count();
        if(h1 != h){
          dp[h1+1][h2] = min( dp[h1+1][h2] , dp[h1][h2] + ccc);
        }
        if(h2 != h){
          dp[h1][h2+1] = min( dp[h1][h2+1] , dp[h1][h2] + ccc);
        }
      }
    }
    ans += dp[h][h];
    // eprintln(dp[h][h]);
  }

  println(ans);

  return 0;
}


Measured memory_bytes: 1144