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

int main() {
	ll H, W, N; cin >> H >> W >> N;
	vector<vector<ll>> dp(H + 1, vector<ll>(W + 1, 0));
	vector<vector<bool>> hole(H + 1, vector<bool>(W + 1, false));
	for (int i = 0; i < N; i++) {
		ll h, w; cin >> h >> w;
		hole[h][w] = true;
	}
	
	
	ll ans = 0;
	for (int h = 1; h <= H; h++) {
		for (int w = 1; w <= W; w++) {
			if (hole[h][w]) continue;
			dp[h][w] = 1 + dp[h-1][w] + dp[h][w-1] - dp[h-1][w-1];
			ans += dp[h][w];
		}
	}
	
	cout << ans << endl;
	
}