#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
const long long int N = 100005;
const long long int MAX = 1e6 + 3;
long long int modPow(long long int a, long long int b) {
  long long int res = 1;
  a %= mod;
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
  }
  return res;
}
long long int nCr(long long int n, long long int k) {
  long long int res = 1;
  if (n < k) return 0;
  if (k > n - k) k = n - k;
  for (long long int i = 0; i < k; ++i) {
    res = (res * (n - i)) % mod;
    res = (res * modPow(i + 1, mod - 2)) % mod;
  }
  return res;
}
void Sieve() {
  set<int> s;
  bool prime[1000009];
  memset(prime, true, sizeof(prime));
  for (long long int p = 2; p * p <= 1000006; p++)
    if (prime[p] == true)
      for (long long int i = p * 2; i <= 1000006; i += p) prime[i] = false;
  for (long long int i = 2; i < 1000006; i++)
    if (prime[i] == true) s.insert(i);
}
long long int factorial(long long int n) {
  if (n == 1) return 1;
  return (n * factorial(n - 1)) % mod;
}
long long int catalan(long long int n) {
  return (nCr(2 * n, n) * modPow(n + 1, mod - 2)) % mod;
}
int main() {
  string s1, s2;
  cin >> s1 >> s2;
  int h = (s1[0] - 48) * 10 + s1[1] - 48;
  int m = (s1[3] - 48) * 10 + s1[4] - 48;
  int time =
      (((s2[0] - 48) * 10 + (s2[1] - 48)) -
       ((s1[0] - 48) * 10 + (s1[1] - 48))) *
          60 +
      (((s2[3] - 48) * 10 + (s2[4] - 48)) - ((s1[3] - 48) * 10 + (s1[4] - 48)));
  time /= 2;
  while (time > 0) {
    m++;
    time--;
    if (m == 60) m = 0, h++;
    if (h == 24) h = 0;
  }
  cout << h / 10 << h % 10 << ":" << m / 10 << m % 10 << "\n";
}
