#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int h1, h2, m1, m2;
  char c;
  cin >> h1 >> c >> m1;
  cin >> h2 >> c >> m2;
  int h_ans;
  h1 = h1 * 60 + m1;
  h2 = h2 * 60 + m2;
  h_ans = (h1 + h2) / 2;
  cout << setw(2) << setfill('0') << h_ans / 60;
  cout << ":";
  cout << setw(2) << setfill('0') << h_ans % 60 << endl;
  ;
  return 0;
}
