# https://yukicoder.me/problems/no/3709


def main():
    H, W, N = map(int, input().split())
    rects = []
    for _ in range(N):
        r1, c1, r2, c2 = map(int , input().split())
        rects.append((r1 - 1, c1 -1, r2 - 1, c2 - 1))

    cum_arrays = [[0] * (W + 1 ) for _ in range(H + 1)]
    for r1, c1, r2, c2 in rects:
        cum_arrays[r1][c1] += 1
        cum_arrays[r1][c2 + 1] -= 1
        cum_arrays[r2 + 1][c1] -= 1
        cum_arrays[r2 + 1][c2 + 1] += 1

    for h in range(H + 1):
        c = 0
        for w in range(W + 1):
            c += cum_arrays[h][w]
            cum_arrays[h][w] = c

    for w in range(W + 1):
        c = 0
        for h in range(H + 1):
            c += cum_arrays[h][w]
            cum_arrays[h][w] = c

    ans = 0
    for h in range(H):
        for w in range(W):
            if cum_arrays[h][w] == 0:
                ans += 1
    print(ans)

    


if __name__ == "__main__":
    main()