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