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

int main(){
    int H, W, Q;
    cin >> H >> W >> Q;
    vector<vector<int>> A(H+1, vector<int>(W+1, 0));

    while(Q--){
        int a, b, c, d;
        cin >> a >> b >> c >> d;
        a--, b--;
        A[a][b]++, A[a][d]--;
        A[c][b]--, A[c][d]++;
    }

    for (int i = 0; i < H; i++){
        for (int j = 0; j < W; j++){
            if (i) A[i][j] += A[i-1][j];
            if (j) A[i][j] += A[i][j-1];
            if (i && j) A[i][j] -= A[i-1][j-1];
        }
    }

    int ans = 0;
    for (int i = 0; i < H; i++){
        for (int j = 0; j < W; j++){
            ans += (A[i][j] == 0);
        }
    }

    cout << ans << endl;
}