#include <bits/stdc++.h>
using namespace std;
int n, m, t[82][82], kezd, veg;
struct Edge {
  int from, to, capacity, cost;
};
vector<Edge> sz;
void add(int a, int b, int c) { sz.push_back({a, b, 1, c}); }
int cs(int a, int b) { return (a - 1) * m + b; }
vector<vector<int>> adj, cost, capacity;
const int INF = 1e9;
void shortest_paths(int n, int v0, vector<int>& d, vector<int>& p) {
  d.assign(n, INF);
  d[v0] = 0;
  vector<bool> inq(n, false);
  queue<int> q;
  q.push(v0);
  p.assign(n, -1);
  while (!q.empty()) {
    int u = q.front();
    q.pop();
    inq[u] = false;
    for (int v : adj[u]) {
      if (capacity[u][v] > 0 && d[v] > d[u] + cost[u][v]) {
        d[v] = d[u] + cost[u][v];
        p[v] = u;
        if (!inq[v]) {
          inq[v] = true;
          q.push(v);
        }
      }
    }
  }
}
int min_cost_flow(int N, vector<Edge> edges, int K, int s, int t) {
  adj.assign(N, vector<int>());
  cost.assign(N, vector<int>(N, 0));
  capacity.assign(N, vector<int>(N, 0));
  for (Edge e : edges) {
    adj[e.from].push_back(e.to);
    adj[e.to].push_back(e.from);
    cost[e.from][e.to] = e.cost;
    cost[e.to][e.from] = -e.cost;
    capacity[e.from][e.to] = e.capacity;
  }
  int flow = 0;
  int cost = 0;
  vector<int> d, p;
  while (flow < K) {
    shortest_paths(N, s, d, p);
    if (d[t] == INF) break;
    int f = K - flow;
    int cur = t;
    while (cur != s) {
      f = min(f, capacity[p[cur]][cur]);
      cur = p[cur];
    }
    flow += f;
    cost += f * d[t];
    cur = t;
    while (cur != s) {
      capacity[p[cur]][cur] -= f;
      capacity[cur][p[cur]] += f;
      cur = p[cur];
    }
  }
  if (flow < K)
    return -1;
  else
    return cost;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin >> n >> m, kezd = n * m + 1, veg = n * m + 2;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      cin >> t[i][j];
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      int x = cs(i, j), y = t[i][j];
      if ((i + j) % 2) {
        add(kezd, x, 0);
        if (i > 1) {
          add(x, cs(i - 1, j), 1 - (y == t[i - 1][j]));
        }
        if (j > 1) {
          add(x, cs(i, j - 1), 1 - (y == t[i][j - 1]));
        }
        if (i < n) {
          add(x, cs(i + 1, j), 1 - (y == t[i + 1][j]));
        }
        if (j < m) {
          add(x, cs(i, j + 1), 1 - (y == t[i][j + 1]));
        }
      } else {
        add(x, veg, 0);
      }
    }
  }
  int ans = min_cost_flow(veg + 1, sz, n * m / 2, kezd, veg);
  cout << ans << endl;
  return 0;
}
