#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string a, b;
  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 min1 = ((h1 * 60) + m1);
  int min2 = ((h2 * 60) + m2);
  int diff = min2 - min1;
  int adder = diff / 2;
  int min3 = min1 + adder;
  int h3 = min3 / 60;
  int m3 = min3 % 60;
  if (h3 / 10 >= 1)
    cout << h3 << ":";
  else
    cout << "0" << h3 << ":";
  if (m3 / 10 >= 1)
    cout << m3;
  else
    cout << "0" << m3;
}
