#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2;
  int m1, m2;
  scanf("%d:%d", &h1, &m1);
  scanf("%d:%d", &h2, &m2);
  int t1 = h1 * 60 + m1;
  int t2 = h2 * 60 + m2;
  int t = t2 - t1;
  t /= 2;
  int tmph = t / 60;
  int tmpm = t % 60;
  int resh, resm;
  resh = h1 + tmph;
  resm = m1 + tmpm;
  if (resm >= 60) {
    resh += resm / 60;
    resm = resm % 60;
    if (resh == 24) resh = 0;
  }
  printf("%02d:%02d\n", resh, resm);
  return 0;
}
