#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int T = 1;
  while (T--) {
    long long int hr1, mn1, hr2, mn2;
    char c;
    cin >> c;
    hr1 = (c - '0') * 10;
    cin >> c;
    hr1 += (c - '0');
    cin >> c;
    cin >> c;
    mn1 = (c - '0') * 10;
    cin >> c;
    mn1 += (c - '0');
    cin >> c;
    hr2 = (c - '0') * 10;
    cin >> c;
    hr2 += (c - '0');
    cin >> c;
    cin >> c;
    mn2 = (c - '0') * 10;
    cin >> c;
    mn2 += (c - '0');
    cin >> c;
    long long int difmin = ((hr2 - hr1) * 60 + (mn2 - mn1)) / 2;
    hr1 += (difmin) / 60;
    mn1 += difmin % 60;
    if (mn1 >= 60) {
      hr1 += mn1 / 60;
      mn1 %= 60;
    }
    if (hr1 < 10)
      cout << 0 << hr1 << ":";
    else
      cout << hr1 << ":";
    if (mn1 < 10)
      cout << 0 << mn1;
    else
      cout << mn1;
  }
  return 0;
}
