#include <bits/stdc++.h>
using namespace std;

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  long long w, h;
  cin >> w >> h;
  int n;
  cin >> n;
  vector<long long> x(n), y(n), a(n), b(n);
  for (int i = 0; i < n; ++i) {
    cin >> x[i] >> y[i];
    --x[i], --y[i];
    a[i] = x[i] - y[i];
    b[i] = x[i] + y[i];
    if ((a[i] - a[0]) & 1) {
      cout << "0\n";
      return 0;
    }
  }
  if (n == 2) {
    if (x[0] > x[1]) {
      swap(x[0], x[1]);
      swap(y[0], y[1]);
    }
    if (y[0] > y[1]) {
      y[0] = h - y[0] - 1;
      y[1] = h - y[1] - 1;
    }
    long long res = 0;
    if (x[0] - y[0] == x[1] - y[1]) {
      res += x[1] - x[0] - 1;
      res += (w - x[1]) * (y[0] + 1);
      res += (x[0] + 1) * (h - y[1]);
    } else {
      if (x[1] - x[0] < y[1] - y[0]) {
        swap(w, h);
        swap(x[0], y[0]);
        swap(x[1], y[1]);
      }
      res += y[1] - y[0] - 1;
      res += y[0] + 1;
      res += h - y[1];
    }
    cout << res << '\n';
    return 0;
  }
  set<long long> sa(begin(a), end(a));
  set<long long> sb(begin(b), end(b));
  long long min_x = *min_element(begin(x), end(x));
  long long max_x = *max_element(begin(x), end(x));
  long long min_y = *min_element(begin(y), end(y));
  long long max_y = *max_element(begin(y), end(y));
  long long min_a = *begin(sa), max_a = *rbegin(sa);
  long long min_b = *begin(sb), max_b = *rbegin(sb);
  long long res = 0;
  if (sa.size() == 1 or sb.size() == 1) {
    res += (w - max_x) * (min_y + 1);
    res += (min_x + 1) * (h - max_y);
  } else if (sa.size() == 2 or sb.size() == 2) {
    long long da = max_a - min_a;
    assert(~da & 1);
    da /= 2;
    long long db = max_b - min_b;
    assert(~db & 1);
    db /= 2;
    if (sa.size() == 2) {
      res += max(db - da + 1, 0LL);
    } else {
      res += max(da - db + 1, 0LL);
    }
  } else {
    for (int i = 0; i < n; ++i) {
      if (a[i] != min_a and a[i] != max_a and
          b[i] != min_b and b[i] != max_b) {
        cout << "0\n";
        return 0;
      }
    }
    res = 1;
  }
  cout << res << '\n';
}