#include <bits/stdc++.h>
using namespace std;
void printNumber(int number) {
  if (number < 10) cout << "0";
  cout << number;
}
int main(void) {
  ios_base::sync_with_stdio(0);
  int h1, h2, m1, m2;
  char bs;
  cin >> h1 >> bs >> m1;
  cin >> h2 >> bs >> m2;
  int minutes1 = h1 * 60 + m1;
  int minutes2 = h2 * 60 + m2;
  int medium = (minutes1 + minutes2) / 2;
  int res_h = medium / 60;
  int res_min = medium % 60;
  printNumber(res_h);
  cout << ":";
  printNumber(res_min);
  cout << "\n";
  return 0;
}
