#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const double EPS = 1e-9;
const int MOD = 1e9 + 7;
int main() {
  int h1, m1, h2, m2;
  scanf("%d:%d", &h1, &m1);
  scanf("%d:%d", &h2, &m2);
  int a = h1 * 60 + m1;
  int b = h2 * 60 + m2;
  int c = (b - a) / 2;
  a += c;
  if ((a / 60) < 10)
    cout << '0' << a / 60 << ':';
  else
    cout << (a / 60) << ':';
  if ((a % 60) < 10)
    cout << '0' << (a % 60) << endl;
  else
    cout << (a % 60) << endl;
  return 0;
}
