H, W, N = map(int, input().split())
X = [[0] * (W+1) for _ in range(H+1)]

for _ in range(N):
    i0, j0, i1, j1 = map(int, input().split())
    X[i0-1][j0-1] += 1
    X[i0-1][j1] -= 1
    X[i1][j0-1] -= 1
    X[i1][j1] += 1

for i in range(H+1):
    for j in range(1,W+1):
        X[i][j] += X[i][j-1]

for j in range(W+1):
    for i in range(1,H+1):
        X[i][j] += X[i-1][j]


ans = 0
for i in range(H):
    for j in range(W):
        if X[i][j] == 0:
            ans += 1

print(ans)