#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
  const auto field = [](const string& s, int i) {
    return (s[i] - '0') * 10 + (s[i + 1] - '0');
  };
  string s, t;
  cin >> s >> t;
  int u = 60 * field(s, 0) + field(s, 3), v = 60 * field(t, 0) + field(t, 3);
  v += u, v /= 2, cout << setw(2) << setfill('0') << (v / 60), cout << ':',
      cout << setw(2) << setfill('0') << (v % 60);
}
