#include <bits/stdc++.h>
int main() {
  int h1, h2, m1, m2;
  scanf("%d:%d %d:%d", &h1, &m1, &h2, &m2);
  m1 += h1 * 60;
  m2 += h2 * 60;
  int ans = (m1 + m2) / 2;
  printf("%02d:%02d", ans / 60, ans % 60);
  return 0;
}
