#include <bits/stdc++.h>
using namespace std;
string a, b;
int main() {
  cin >> a >> b;
  int h1 = (a[0] - '0') * 10 + (a[1] - '0');
  int m1 = (a[3] - '0') * 10 + (a[4] - '0');
  int h2 = (b[0] - '0') * 10 + (b[1] - '0');
  int m2 = (b[3] - '0') * 10 + (b[4] - '0');
  int k = h2 - h1;
  int t = m2 - m1;
  int totalMins = (k * 60 + t) / 2;
  h1 = (h1 + totalMins / 60);
  m1 = (m1 + totalMins % 60);
  if (m1 >= 60) {
    h1++;
    m1 %= 60;
  }
  if (h1 < 10) {
    cout << 0;
  }
  cout << h1 << ':';
  if (m1 < 10) {
    cout << 0;
  }
  cout << m1;
  cout << '\n';
  return 0;
}
