#include <bits/stdc++.h>
int main() {
  int h1, h2, m1, m2;
  scanf("%d:%d %d:%d", &h1, &m1, &h2, &m2);
  int t = (h1 * 60 + m1) + (h2 * 60 + m2);
  int t1 = t / 2;
  int h3 = t1 / 60;
  int m3 = t1 % 60;
  printf("%02d:%02d\n", h3, m3);
}
