#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  char s;
  cin >> a >> s >> b >> c >> s >> d;
  int x = a + c, y = b + d;
  if (x % 2 == 1) {
    x -= 1;
    y += 60;
  }
  x /= 2;
  y /= 2;
  if (y >= 60) {
    x += 1;
    y -= 60;
  }
  if (x < 10)
    cout << 0 << x;
  else
    cout << x;
  cout << ":";
  if (y < 10)
    cout << 0 << y << endl;
  else
    cout << y << endl;
}
