#include <iostream>
using namespace std;
bool a[51][51] = { 0 };
int bi(int x, int y)
{
    a[x][y] = 1;
    if (a[x - 1][y] == 0){
        a[x-1][y]=1;
        return 1;
    }
    if (a[x][y - 1] == 0){
        a[x][y-1]=1;
        return 1;
    }
    a[x][y] = 0;
    return 0;
}
int main()
{
    int m, n, k, ans = 0;
    cin >> m >> n >> k;
    for (int i = 1; i <= k; i++) {
        int x, y;
        cin >> x >> y;
        a[x][y] = 1;
    }
    for (int i = 0; i <= n; i++) {
        a[0][i] = 1;
    }
    for (int i = 1; i <= m; i++) {
        a[i][0] = 1;
    }
    for (int i = 1; i <= m; i++) {
        for (int j = 1; j <= n; j++) {
            if (a[i][j] !=1) {
                ans += bi(i, j);
            }/*
            cout<<ans<<endl;
            for (int u = 0; u <= m; u++) {
                for (int e = 0; e <= n; e++) {
                    cout << a[u][e] << " ";
                }
                cout << endl;
            }
            cout << endl
                 << endl;*/
        }
    }
    cout << ans;
}