#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  string s1, s2;
  cin >> s1 >> s2;
  string r = s1.substr(0, 2);
  string r1 = s1.substr(3, 2);
  string r2 = s2.substr(0, 2);
  string r3 = s2.substr(3, 2);
  int k = 0;
  int k1 = 0;
  int k2 = 0;
  int k3 = 0;
  k = ((r[0] - '0') * 10) + r[1] - '0';
  k1 = ((r1[0] - '0') * 10) + r1[1] - '0';
  k2 = ((r2[0] - '0') * 10) + r2[1] - '0';
  k3 = ((r3[0] - '0') * 10) + r3[1] - '0';
  int r9 = (k * 60) + k1;
  int r11 = (k2 * 60) + k3;
  int u = 0;
  if (r11 < r9) {
    u = (((24 * 60) - r9) + r11) / 2;
  } else
    u = abs(r11 - r9) / 2;
  int u1 = 0;
  int u2 = 0;
  u1 = u + k1;
  if (u + k1 >= 60) {
    int o = (u + k1) / 60;
    u2 += o;
    u1 = u + k1 - o * 60;
  }
  if (k + u2 < 10)
    cout << 0 << k + u2 << ":";
  else if (k + u2 <= 23)
    cout << k + u2 << ":";
  else {
    if (k + u2 - 24 > 10)
      cout << k + u2 - 24 << ":";
    else
      cout << 0 << k + u2 - 24 << ":";
  }
  if (u1 < 10) {
    cout << 0 << u1;
  } else {
    cout << u1;
  }
}
