codeforces 1133A in python
sh, sm = map(int, input().split(':')) # start time
eh, em = map(int, input().split(':')) # end time
mid_m = (((sh * 60) + sm) + ((eh * 60) + em)) // 2
if mid_m >= 1440:
mid_m = mid_m - 1440
mh = mid_m // 60
mm = mid_m - (mh * 60)
if mh < 10:
mh = '0' + str(mh)
if mm < 10:
mm = '0' + str(mm)
print(mh, ':', mm, sep='')