from sys import stdin
input = stdin.readline

H, W, N = map(int, input().split())
A = [list(map(int, input().split())) for _ in range(N)]

imos = [[0]*(W+1) for _ in range(H+1)]
for a, b, c, d in A:
    a, b, c, d = a-1, b-1, c-1, d-1
    imos[a][b] += 1
    imos[a][d+1] -= 1
    imos[c+1][b] -= 1
    imos[c+1][d+1] += 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)))