#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2, m1, m2;
  scanf("%d:%d", &h1, &m1);
  scanf("%d:%d", &h2, &m2);
  int h = h1 + h2;
  int m = m1 + m2;
  if (m >= 60) {
    m -= 60, h += 1;
  }
  if (h % 2 == 0) {
    printf("%02d:%02d\n", h / 2, m / 2);
  } else {
    h -= 1;
    m += 60;
    printf("%02d:%02d\n", h / 2, m / 2);
  }
  return 0;
}
