h1, m1 = map(int, input().split(':'))
h2, m2 = map(int, input().split(':'))
h3, m3 = map(int, input().split(':'))
h4, m4 = map(int, input().split(':'))

t1 = h1 * 60 + m1
t2 = h2 * 60 + m2
t3 = h3 * 60 + m3
t4 = h4 * 60 + m4

AB_time = t2 - t1
BA_time = t4 - t3

double_time = AB_time + BA_time
real_time = (double_time + double_time % 2) // 2
print(f'{real_time // 60:02}', f'{real_time % 60:02}', sep=':')