#include <bits/stdc++.h>
const int Max_N = 1e3 + 6;
using namespace std;
int main() {
  int h1, h2, m1, m2;
  char c;
  cin >> h1 >> c >> m1;
  cin >> h2 >> c >> m2;
  int mid = (h2 * 60 + m2) - (h1 * 60 + m1);
  mid /= 2;
  h2 = (h1 * 60 + m1 + mid) / 60;
  m2 = (h1 * 60 + m1 + mid) % 60;
  printf("%02d:%02d", h2, m2);
}
