data = input()
data = data.split()
for i in range(len(data)):
    data[i] = int(data[i])
#print(data)
range1 = data[1] - data[0] + 1
range2 = data[3] - data[2] + 1
sa = data[1] - data[2] + 1
if sa < 0:
    sa = 0
#print(sa)
#print(range1)
#print(range2)
print(range1 * range2 - sa)