#include <bits/stdc++.h>
using namespace std;
int a, b, c, d;
string s1, s2;
int main() {
  cin >> s1 >> s2;
  string a1 = s1.substr(0, 2), a2 = s1.substr(3, 2);
  string b1 = s2.substr(0, 2), b2 = s2.substr(3, 2);
  int a = atoi(a1.c_str()), b = atoi(a2.c_str()), c = atoi(b1.c_str()),
      d = atoi(b2.c_str());
  int hours, mins;
  if ((a + c) % 2 == 0) {
    mins = (b + d) / 2;
  } else {
    mins = (b + d) / 2 + 30;
  }
  if (mins >= 60) {
    hours = (a + c) / 2 + 1;
    mins = mins % 60;
  } else {
    hours = (a + c) / 2;
  }
  cout << ((hours) < 10 ? "0" : "") << hours << ":" << ((mins) < 10 ? "0" : "")
       << mins << endl;
  return 0;
}
