#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 100;
const int MAX_M = 10000;
int ha, hb, ma, mb;
int main() {
  scanf("%d:%d", &ha, &ma);
  scanf("%d:%d", &hb, &mb);
  if (ha == hb) {
    printf("%02d:%02d\n", ha, (ma + mb) / 2);
  } else {
    int hc = ha;
    int len = mb + (hb - ha) * 60 - ma;
    len /= 2;
    len += ma;
    while (len >= 60) {
      hc++;
      len -= 60;
    }
    printf("%02d:%02d\n", hc, len);
  }
  return 0;
}
