#include <bits/stdc++.h>
using namespace std;
int main() {
  char t1[5];
  char t2[5];
  for (int i = 0; i < 5; i++) {
    cin >> t1[i];
  }
  for (int i = 0; i < 5; i++) {
    cin >> t2[i];
  }
  int rh, rm;
  int h1 = (t1[0] - 48) * 10 + t1[1] - 48;
  int m1 = (t1[3] - 48) * 10 + t1[4] - 48;
  int h2 = (t2[0] - 48) * 10 + t2[1] - 48;
  int m2 = (t2[3] - 48) * 10 + t2[4] - 48;
  int tm1, tm2;
  tm1 = h1 * 60 + m1;
  tm2 = h2 * 60 + m2;
  int m = (tm2 - tm1) / 2;
  rh = h1 + m / 60;
  rm = m1 + m % 60;
  if (rm > 59) {
    rh++;
    rm = rm - 60;
  }
  if (rh < 10) {
    cout << 0 << rh << ":";
  } else
    cout << rh << ":";
  if (rm < 10) {
    cout << 0 << rm;
  } else
    cout << rm;
}
