#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int h1, m1, h2, m2;
  string s1, s2;
  cin >> s1;
  cin >> s2;
  h2 = (s2[0] - '0') * 10 + (s2[1] - '0');
  m1 = (s1[3] - '0') * 10 + (s1[4] - '0');
  h1 = h1 = (s1[0] - '0') * 10 + (s1[1] - '0');
  m2 = (s2[3] - '0') * 10 + (s2[4] - '0');
  long long int tot = (h2 - h1) * 60 + (m2 - m1);
  tot /= 2;
  long long int a = tot / 60;
  long long int b = tot % 60;
  if (b + m1 >= 60) {
    a++;
    b -= 60;
  }
  if (a + h1 < 10) cout << 0;
  cout << a + h1 << ":";
  if (b + m1 < 10) cout << 0;
  cout << b + m1;
}
