#include <bits/stdc++.h>
using namespace std;
int m1, n1, m2, n2;
int main() {
  scanf("%d:%d%d:%d", &m1, &n1, &m2, &n2);
  int ans = (m1 * 60 + m2 * 60 + n1 + n2) / 2;
  int num1 = ans / 60;
  int num2 = ans % 60;
  if (num1 < 10) {
    cout << 0 << num1;
  } else {
    cout << num1;
  }
  cout << ":";
  if (num2 < 10) {
    cout << 0 << num2;
  } else {
    cout << num2;
  }
  return 0;
}
