#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string start;
  string end;
  cin >> start;
  cin >> end;
  int startInt = (stoi(start.substr(0, 3))) * 60 + (stoi(start.substr(3, 5)));
  int endInt = (stoi(end.substr(0, 3))) * 60 + (stoi(end.substr(3, 5)));
  int dif = (endInt - startInt) / 2;
  int hoursToAdd = dif / 60;
  int minutesToAdd = dif % 60;
  int first = (stoi(start.substr(0, 3)) + hoursToAdd);
  int second = (stoi(start.substr(3, 5)) + minutesToAdd);
  first += second / 60;
  second = second % 60;
  string firstS = to_string(first);
  string secondS = to_string(second);
  if (firstS.size() == 1) firstS.insert(0, "0");
  if (secondS.size() == 1) secondS.insert(0, "0");
  cout << firstS << ':' << secondS << "\n";
}
