#include <bits/stdc++.h>
using namespace std;
int h1, h2, m1, m2, total, h, m;
int main() {
  scanf("%d:%d", &h1, &m1);
  scanf("%d:%d", &h2, &m2);
  if (h1 < h2) {
    if (m1) total += 60 - m1, h1++;
    total += 60 * (h2 - h1);
    total += m2;
    if (m1) h1--;
  } else if (h1 > h2) {
    if (m2) h2++, total += 60 - m2;
    total += 60 * (12 - (h2 - h1));
    total += m1;
    if (m2) h2--;
  } else {
    if (m1 <= m2) {
      total += m2 - m1;
    } else {
      total += 12 * 60;
      total -= (m2 - m1);
    }
  }
  total /= 2;
  m = m1 + total;
  h = m / 60 + h1;
  m %= 60;
  if (h / 10 == 0) cout << 0;
  cout << h << ':';
  if (m / 10 == 0) cout << 0;
  cout << m << '\n';
  return 0;
}
