#task - №133
n, m = [int(x) for x in input().split()]
row, col = [int(x) for x in input().split()]

if n < 1 or m < 1 or row > n or col > m or row < 1 or col < 1:
	print('invalid input params')
	exit(0)


r = (row - 1) * m
if row % 2 == 0:
	r += (m - col + 1)
else:
	r += col
	
print(r)