#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, m1, h2, m2;
  string s1, s2;
  cin >> s1 >> s2;
  h1 = (s1[0] - '0') * 10 + s1[1] - '0';
  m1 = (s1[3] - '0') * 10 + s1[4] - '0';
  h2 = (s2[0] - '0') * 10 + s2[1] - '0';
  m2 = (s2[3] - '0') * 10 + s2[4] - '0';
  int x = (h2 - h1) * 60;
  x = x - m1 + m2;
  x = x / 2;
  int h = h1 + x / 60;
  int v = x % 60;
  int count = v;
  int m = m1;
  while (count--) {
    m++;
    if (m == 60) {
      h++;
      m = 0;
    }
  }
  string w = to_string(h);
  string w2 = to_string(m);
  if (w.length() == 1) w = "0" + w;
  if (w2.length() == 1) w2 = "0" + w2;
  cout << w << ":" << w2 << endl;
}
