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