H, W, N = map(int, input().split())

B = [[0] * (W + 1) for i in range(H + 1)]
for _ in range(N):
	r1, c1, r2, c2 = map(int, input().split()); r1 -= 1; c1 -= 1
	B[r1][c1] += 1; B[r1][c2] -= 1; B[r2][c1] -= 1; B[r2][c2] += 1
for i in range(H):
	for j in range(W + 1):
		B[i + 1][j] += B[i][j]
for i in range(H + 1):
	for j in range(W):
		B[i][j + 1] += B[i][j]

print(sum(B[i][j] == 0 for i in range(H) for j in range(W)))