#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    int h, w, n;
    cin >> h >> w >> n;
    vector a(h + 1, vector<int>(w, 0));
    rep(i, n) {
        int r1, c1, r2, c2;
        cin >> r1 >> c1 >> r2 >> c2;
        r1--;
        c1--;
        a[r2][c2]++;
        a[r1][c2]--;
        a[r2][c1]--;
        a[r1][c1]++;
    }
    rep(i, h) rep(j, w + 1) {
        a[i + 1][j] += a[i][j];
    }
    rep(i, h + 1) rep(j, w) {
        a[i][j + 1] += a[i][j];
    }
    int ans = 0;
    rep(i, h) rep(j, w) {
        if (a[i][j] == 0)
            ans++;
    }
    cout << ans << endl;
    return 0;
}