package main

import (
	"fmt"
)

func main() {
	var s1, s2 string
	fmt.Scan(&s1, &s2)
	var h1, m1, h2, m2 int
	fmt.Sscanf(s1, "%d:%d", &h1, &m1)
	fmt.Sscanf(s2, "%d:%d", &h2, &m2)
	t := (h1*60 + m1 + h2*60 + m2) / 2
	fmt.Printf("%02d:%02d\n", t/60, t%60)
}