#include <bits/stdc++.h>
using namespace std;
int main() {
  string s1, s2;
  cin >> s1 >> s2;
  int h1 = (s1[0] - '0') * 10 + (s1[1] - '0');
  int m1 = (s1[3] - '0') * 10 + (s1[4] - '0');
  int h2 = (s2[0] - '0') * 10 + (s2[1] - '0');
  int m2 = (s2[3] - '0') * 10 + (s2[4] - '0');
  int hh = h2 - h1;
  int mm = m2 - m1;
  int diff = hh * 60 + mm;
  diff = diff / 2;
  int h_ans = (diff / 60) + h1;
  int m_ans = (diff % 60) + m1;
  if (m_ans >= 60) {
    m_ans = m_ans % 60;
    h_ans++;
  }
  if (h_ans / 10 == 0) cout << "0";
  cout << h_ans << ":";
  if (m_ans / 10 == 0) cout << "0";
  cout << m_ans << "\n";
}
