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