#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2, m1, m2, x1, x2, x3, x4, x5, x6;
  char c;
  cin >> h1 >> c >> m1;
  cin >> h2 >> c >> m2;
  x1 = 60 * h1 + m1;
  x2 = 60 * h2 + m2;
  x3 = (x2 - x1) / 2;
  x4 = x1 + x3;
  x6 = x4 % 60;
  x5 = x4 / 60;
  if (x5 < 10 && x6 < 10)
    cout << "0" << x5 << ":"
         << "0" << x6;
  else if (x5 < 10 && x6 >= 10)
    cout << "0" << x5 << ":" << x6;
  else if (x5 >= 10 && x6 < 10)
    cout << x5 << ":"
         << "0" << x6;
  else if (x5 >= 10 && x6 >= 10)
    cout << x5 << ":" << x6;
}
