#include <bits/stdc++.h>
using namespace std;
bool func(int x) {
  int cnt = 0;
  while (x != 0) {
    x = x / 10;
    cnt++;
  }
  if (cnt == 1)
    return true;
  else if (cnt == 2)
    return false;
  else if (cnt == 0)
    return true;
}
int main() {
  int h1, h2, m1, m2;
  int h, m, temp;
  char ch;
  cin >> h1;
  cin >> ch;
  cin >> m1;
  cin >> h2;
  cin >> ch;
  cin >> m2;
  int dh, dm, t;
  dh = h2 - h1;
  dm = m2 - m1;
  if (dm == 0) {
    float g = (double)dh / 2;
    g *= 60;
    if (g >= 60) {
      int ho = g / 60;
      int mi = (long long)g % 60;
      h = h1 + ho;
      m = mi + m1;
    } else {
      m = m1 + g;
      if (m >= 60) {
        h = h1 + (m / 60);
        m = m % 60;
      } else
        h = h1;
    }
  } else if (dh == 0) {
    h = h1;
    m = m1 + dm / 2;
  } else {
    if (dm < 0) {
      --dh;
      t = dh + (60 - m1) + m2;
    } else {
      dh;
      t = dh * 60 + (m2 - m1);
    }
    t = t / 2;
    if (t + m1 >= 60) {
      int u = h1 + t / 60;
      int left = t % 60;
      if (m1 + left >= 60) {
        ++u;
        m = (left - (60 - m1));
      } else
        m = m1 + left;
      h = u;
    } else {
      h = h1;
      m = m1 + t;
    }
  }
  if (func(h) == true && func(m) == true) {
    cout << '0' << h << ":" << '0' << m << endl;
  } else if (func(h) && !func(m)) {
    cout << '0' << h << ":" << m << endl;
  } else if (!func(h) && func(m)) {
    cout << h << ":" << '0' << m << endl;
  } else
    cout << h << ":" << m << endl;
}
