#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)
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