# coding=utf-8:

a = list(map(int, input().split()))
b = list(map(int, input().split()))

m_a = abs(a[0]) + abs(a[1])
m_b = abs(b[0]) + abs(b[1])

if (m_a + m_b) % 2 == 0:
    m_ab = (m_a + m_b) // 2

else:
    m_ab = (m_a + m_b) / 2

print(m_ab)