h, w, n = list(map(int, input().split()))
A = [[0 for _ in range(w+2)] for _ in range(h+2)]
for _ in range(n):
    y0, x0, y1, x1 = list(map(lambda x: int(x)-1, input().split()))
    A[y0][x0] += 1
    A[y0][x1+1] -= 1
    A[y1+1][x0] -= 1
    A[y1+1][x1+1] += 1

ans = 0
for y in range(h):
    for x in range(w):
        A[y][x] += A[y-1][x] + A[y][x-1] - A[y-1][x-1]
        if not A[y][x]: ans += 1
print(ans)