#include <bits/stdc++.h>
int main() {
  int total, total1, total2, hr, minute;
  char str1[10];
  char str2[10];
  scanf("%[^\n]%*c", str1);
  scanf("%[^\n]%*c", str2);
  total1 = ((str1[0] - '0') * 10 + (str1[1] - '0')) * 60 +
           ((str1[3] - '0') * 10 + (str1[4] - '0'));
  total2 = ((str2[0] - '0') * 10 + (str2[1] - '0')) * 60 +
           ((str2[3] - '0') * 10 + (str2[4] - '0'));
  total = total2 - total1;
  total = total / 2;
  hr = total / 60;
  minute = total - (hr * 60);
  hr = ((str1[0] - '0') * 10 + (str1[1] - '0')) + hr;
  minute = ((str1[3] - '0') * 10 + (str1[4] - '0')) + minute;
  if (minute >= 60) {
    hr = hr + 1;
    minute = minute - 60;
  }
  if (hr <= 9 && minute <= 9) {
    printf("0%d:0%d\n", hr, minute);
  } else if (hr <= 9 && minute > 9) {
    printf("0%d:%d\n", hr, minute);
  } else if (hr > 9 && minute <= 9) {
    printf("%d:0%d\n", hr, minute);
  } else if (hr > 9 && minute > 9) {
    printf("%d:%d\n", hr, minute);
  }
}
