#include <bits/stdc++.h>
using namespace std;
int main() {
  int s1h, s2h, s1m, s2m, hour, min, totmin, fhour = 0;
  char c;
  cin >> s1h >> c >> s1m;
  cin >> s2h >> c >> s2m;
  hour = (s2h - s1h) - 1;
  min = (60 - s1m) + s2m;
  totmin = (((60 * hour) + min) / 2);
  while (totmin >= 60) {
    totmin = totmin - 60;
    fhour++;
  }
  if ((s1m + totmin) >= 60) {
    s1h = s1h + 1;
    totmin = (s1m + totmin) - 60;
    cout << setfill('0') << setw(2) << (s1h + fhour) << ":" << setfill('0')
         << setw(2) << totmin;
  } else {
    cout << setfill('0') << setw(2) << (s1h + fhour) << ":" << setfill('0')
         << setw(2) << (totmin + s1m);
  }
  return 0;
}
