#include <bits/stdc++.h>
using namespace std;
vector<vector<char>> dp;
int n, m, k;
int check(int l, int r) {
vector<int> mas(m + 1, 0);
for (int j = 1; j <= m; j++) {
for (int i = l; i <= r; i++) {
mas[j] |= dp[i][j];
}
}
return (count(mas.begin(), mas.end(), 0) == 1 ? 1 : 0);
}
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m >> k;
dp = vector<vector<char>> (1 + n, vector<char>(1 + m, 0));
for (int i = 0; i < k; i++) {
int a, b;
cin >> a >> b;
dp[a][b] = 1;
}
int ans = 0;
for (int l = 1; l <= n; l++) {
for (int r = l + m - 1; r <= n; r++) {
if (check(l, r)) {
ans ++;
}
}
}
cout << ans << '\n';
return 0;
}
