#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#include <tuple>
#include <iomanip>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<ll> vec;
typedef vector<vec> mat;
#define rep(i, n) for(ll i = 0; i < (n); i++)
#define revrep(i, n) for(ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
/*
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll max3(ll a, ll b, ll c){return max(a, max(b, c));};
ll min3(ll a, ll b, ll c){return min(a, min(b, c));};
ll max4(ll a, ll b, ll c, ll d){return max(max(a, b), min(c, d));};
ll min4(ll a, ll b, ll c, ll d){return min(min(a, b), min(c, d));};
ll max5(ll a, ll b, ll c, ll d, ll e){return max(max(a, b), max3(c, d, e));};
ll min5(ll a, ll b, ll c, ll d, ll e){return min(min(a, b), min3(c, d, e));};
*/
const ll INFL = 1LL << 60;//10^18 = 2^60
const int INF = 1 << 30;//10^9
ll MOD = 1000000007;
//ll MOD = 998244353;

vector<ll> dy = {0, 0, 1, -1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, -1, 0, 0, 1, -1, 1, -1, 0};


ll pow_long(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2) res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}
ll pow_mod(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2){
      res *= x; res %= MOD;
    }
    x *= x; x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x){return pow_mod(x, MOD - 2);};

ll gcd(ll a, ll b){
    if(b == 0) return a;
    return gcd(b, a % b);
}

ll lcm(ll x, ll y){return x / gcd(x, y) * y;};

ll kai_mod(ll x){
  if(x == 0) return 1;
  return x * kai_mod(x-1) % MOD;
}


//コンビネーション
const int MAXcomb = 200010;
ll fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
//facはn!,finvは1/n!
//invは逆元
void COMinit(){
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i = 2; i < MAXcomb; i++){
        fac[i] = fac[i-1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD/i) % MOD;
        finv[i] = finv[i-1] * inv[i] % MOD;
    }
}
ll comb(int n, int k){
    if(n < k) return 0;
    if(n < 0 || k < 0) return 0;
    return fac[n] * finv[k] % MOD * finv[n-k] % MOD;
}

ll H, W;
char tmp[310][310];
string Q[310];
ll subsubsolve(ll a, ll sl, ll tl){
  string S = Q[a], T = Q[a+1];
  string s, t;
  if(sl >= tl){
    s = S.substr(sl - tl, tl);
    t = T.substr(0, tl);
  }else{
    s = S.substr(0, sl);
    t = T.substr(tl - sl, sl);
  }
  ll res = 0;
  rep(i, min(sl, tl)){
    if(s[i] == t[i]) res++;
  }
  return res;
}

ll memo[310][310][310];

ll saiki(ll a, ll i, ll j){
  if(i == H || j == H) return memo[a][i][j] = 0;
  if(memo[a][i][j] != -1) return memo[a][i][j];
  return memo[a][i][j] = saiki(a, i+1, j+1) + (Q[a][i] == Q[a+1][j]);
}

void make_memo(ll a){
  rep(i, 310)rep(j, 310) memo[a][i][j] = -1;
  rep(i, H) saiki(a, 0, i);
  rep(i, H) saiki(a, i, 0);
}


ll subsolve(ll a){
  make_memo(a);
  string S = Q[a], T = Q[a+1];
  ll dp[H + 1][H + 1];
  rep(i, H + 1)rep(j, H + 1) dp[i][j] = INFL;
  dp[H][H] = 0;
  for(ll i = H; i >= 0; i--){
    for(ll j = H; j >= 0; j--){
      ll cnt;
      if(i >= j) cnt = memo[a][i-j][0] - memo[a][i][j];
      else cnt = memo[a][0][j-i] - memo[a][i][j];
      if(i != 0) dp[i-1][j] = min(dp[i-1][j], dp[i][j] + cnt);
      if(j != 0) dp[i][j-1] = min(dp[i][j-1], dp[i][j] + cnt);
    }
  }
  return dp[0][0];
}

void solve(){
  ll ans = 0;
  rep(i, W-1){
    ans += subsolve(i);
  }
  cout << ans << endl;
}

int main(){
  cin >> H >> W;
  rep(i, H){
    rep(j, W) cin >> tmp[i][j];
  }
  rep(i, H){
    rep(j, W) Q[j] += tmp[i][j];
  }

  solve();

}


Measured memory_bytes: 228480