#include <bits/stdc++.h>
using namespace std;
int main() {
  string start, end;
  cin >> start >> end;
  int sh = stoi(start.substr(0, 2)), sm = stoi(start.substr(3, 2));
  int eh = stoi(end.substr(0, 2)), em = stoi(end.substr(3, 2));
  int st = sh * 60 + sm;
  int et = eh * 60 + em;
  int mid = (st + et) / 2;
  cout << (mid / 60 < 10 ? "0" : "") << mid / 60 << ":"
       << (mid % 60 < 10 ? "0" : "") << mid % 60 << endl;
  return 0;
}
