#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  string a, b;
  cin >> a >> b;
  auto parse = [&](const string& s) {
    int h = (s[0] - '0') * 10 + (s[1] - '0');
    int m = (s[3] - '0') * 10 + (s[4] - '0');
    return make_pair(h, m);
  };
  auto n = parse(a);
  auto m = parse(b);
  int ha = n.first, ma = n.second;
  int hb = m.first, mb = m.second;
  if (ha > hb || (ha == hb && ma > mb)) {
    swap(ha, hb);
    swap(ma, mb);
  }
  int hs = 0, ms = 0;
  if (ha == hb) {
    hs = ha;
    ms = ma + (mb - ma) / 2;
  } else {
    int k = (60 - ma) + (hb - ha - 1) * 60 + mb;
    k /= 2;
    hs = ha + k / 60;
    ms = ma + k % 60;
    if (ms >= 60) {
      ms -= 60;
      ++hs;
    }
    hs %= 24;
    ms %= 60;
  }
  if (hs < 10) cout << "0";
  cout << hs << ":";
  if (ms < 10) cout << "0";
  cout << ms << "\n";
}
