#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, e;
  cin >> s >> e;
  int h1 = stoi(s.substr(0, 2)), m1 = stoi(s.substr(3, 2)),
      h2 = stoi(e.substr(0, 2)), m2 = stoi(e.substr(3, 2));
  if (h1 == h2) {
    cout << h1 / 10 << h1 % 10 << ':' << (m1 + m2) / 20 << ((m1 + m2) / 2) % 10
         << endl;
  } else {
    int elapsed_m = 60 - m1 + m2 + (h2 - h1 - 1) * 60;
    int h3 = h1 + (m1 + elapsed_m / 2) / 60, m3 = (m1 + elapsed_m / 2) % 60;
    cout << h3 / 10 << h3 % 10 << ':' << m3 / 10 << m3 % 10 << endl;
  }
  return 0;
}
