#include <bits/stdc++.h>
using namespace std;
const double PI = atan(1.0) * 4;
const int INF = 0x7ffffff;
const long long MOD = 1000000007;
const int maxn = 200010;
int main() {
  char s[10];
  int a = 0, b = 0, c, ans1, ans2;
  scanf("%s", s);
  a = (s[0] - '0') * 10 + (s[1] - '0');
  a = a * 60 + (s[3] - '0') * 10 + (s[4] - '0');
  scanf("%s", s);
  b = (s[0] - '0') * 10 + (s[1] - '0');
  b = b * 60 + (s[3] - '0') * 10 + (s[4] - '0');
  c = (a + b) / 2;
  ans1 = c / 60;
  ans2 = c % 60;
  if (ans1 < 10) printf("0");
  printf("%d:", ans1);
  if (ans2 < 10) printf("0");
  printf("%d\n", ans2);
  return 0;
}
