E

#include <bits/stdc++.h>

using namespace std;

#define ll     long long
#define _test   int _TEST; cin>>_TEST; while(_TEST--)
#define ff     first
#define ss     second
#define pb     push_back

int main()
{
    int n, m;
    cin>>n>>m;

    vector<vector<int>> a(n, vector<int> (m));

    int N;
    cin>>N;

    for(int i=0; i<N; i++)
    {
        int x, y;
        cin>>x>>y;

        x--, y--;

        a[x][y] = 1;
    }

    vector<vector<ll int>> vals(n, vector<ll int> (m));
    ll int ans = 0;

    for(int i=0; i<n; i++)
    {
        for(int j=0; j<m; j++)
        {
            if(i==0 || j==0)
            {
                if(a[i][j] == 1)    vals[i][j] = 0;
                else                vals[i][j] = 1;
            }
            else
            {
                if(a[i][j] == 1)    vals[i][j] = 0;
                else                vals[i][j] = min({vals[i-1][j], vals[i][j-1], vals[i-1][j-1]}) + 1;
            }

            ans += vals[i][j];
        }
    }

    cout<<ans<<"\n";
}