#include <bits/stdc++.h>

#if defined(LOCAL)
#define DBG_MACRO_NO_WARNING
#include <dbg.hpp>
#else
#define dbg(x...) (0)
#endif

using namespace std;

using ll = long long;

// #define int ll
#define rep(i, f, t) for (int i = (f), ed##i = (t); i <= ed##i; ++i)
#define re(i, t) rep (i, 1, t)
#define per(i, t, f) for (int i = (t), ed##i = (f); i >= ed##i; --i)
#define ste(i, f, t, s) for (int i = (f), ed##i = (t); i <= ed##i; i += s)
#define each(i, x) for (auto &&i : (x))
#define nxt(i, f, g) for (int i = g.h[f]; i; i = g.e[i].n)
#define umod(x) ((x) >= mo && ((x) -= mo))
#define dmod(x) ((x) < 0 && ((x) += mo))
#define y1 y1__
#define fio(x) (freopen(x ".in", "r", stdin), freopen(x ".out", "w", stdout))

template <class T, class E>
__attribute__((always_inline)) inline void up(T &x, E &&y) {
  if (x < y) x = y;
}
template <class T, class E>
__attribute__((always_inline)) inline void down(T &x, E &&y) {
  if (y < x) x = y;
}

const int N = 1e5 + 9;

struct G {
  int tot = 1, h[N];
  struct E {
    int t, n;
    int ca, fl, co;
  } e[N << 1];

  inline void Add(int f, int t, int ca, int fl, int co) {
    e[++tot] = {t, h[f], ca, fl, co}, h[f] = tot;
  }
} g;

int n, m, S, T, cur[N], q;
int dis[N], co;
bool inq[N], vis[N];

inline int Id(int x, int y, int b) { return b * n * m + (x - 1) * m + y; }

inline void Add(int f, int t, int ca, int co) { g.Add(f, t, ca, 0, co), g.Add(t, f, 0, 0, -co); }

inline bool Bfs() {
  re (i, T) inq[i] = 0, dis[i] = 0x3f3f3f3f;
  inq[S] = 1, dis[S] = 0;
  queue<int> q;
  q.push(S);
  while (q.size()) {
    int f = q.front();
    q.pop();
    inq[f] = 0;
    nxt (i, f, g) {
      int t = g.e[i].t, v = g.e[i].co;
      if (g.e[i].fl >= g.e[i].ca) continue;
      if (dis[f] + v < dis[t]) {
        dis[t] = dis[f] + v;
        if (!inq[t]) q.push(t), inq[t] = 1;
      }
    }
  }
  return dis[T] < 0x3f3f3f3f;
}

inline int Dfs(int f, int fl) {
  if (!fl || f == T) return fl;
  int ou = 0, tmp;
  vis[f] = 1;
  for (int &i = cur[f]; i; i = g.e[i].n) {
    int t = g.e[i].t;
    if (!vis[t] && dis[t] == dis[f] + g.e[i].co && g.e[i].fl <= g.e[i].ca &&
        (tmp = Dfs(t, min(fl, g.e[i].ca - g.e[i].fl))))
      ou += tmp, fl -= tmp, g.e[i].fl += tmp, g.e[i ^ 1].fl -= tmp, co += g.e[i].co * tmp;
    if (!fl) break;
  }
  if (!ou) dis[f] = 0x3f3f3f3f;
  vis[f] = 0;
  return ou;
}

inline int Dinic() {
  int fl = 0;
  while (Bfs()) {
    re (i, T) cur[i] = g.h[i];
    int tmp;
    while ((tmp = Dfs(S, 1e9))) {
      fl += tmp;
    }
  }
  return fl;
}

inline void Work() {
  S = 2 * n * m + 1, T = S + 1;
  g.tot = 1;
  rep (i, 0, T) g.h[i] = 0;
  co = 0;
  re (i, q) {
    int x, y;
    cin >> x >> y;
    Add(S, Id(x, y, 0), 1, 0);
  }
  int cnt = 0;
  re (i, n)
    re (j, m)
      if (i == 1 || j == 1 || i == n || j == m) Add(Id(i, j, 1), T, 1, 0), ++cnt;
  if (cnt < q) return cout << "-1\n", void();
  re (i, n)
    re (j, m) {
      Add(Id(i, j, 0), Id(i, j, 1), 1, 0);
      if (i != 1) Add(Id(i, j, 1), Id(i - 1, j, 0), 1, 1);
      if (i != n) Add(Id(i, j, 1), Id(i + 1, j, 0), 1, 1);
      if (j != 1) Add(Id(i, j, 1), Id(i, j - 1, 0), 1, 1);
      if (j != m) Add(Id(i, j, 1), Id(i, j + 1, 0), 1, 1);
    }
  int fl = Dinic();
  if (fl != q)
    cout << "-1\n";
  else
    cout << co << '\n';
}

signed main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  while (cin >> n >> m >> q) Work();
  return 0;
}