text: #include<bits/stdc++.h>
  using namespace std;
  int main()
  {
  	ios_base::sync_with_stdio(false);
  	cin.tie(NULL);
  	int i,j;
  	char ch;
  	string a,b;
  	cin>>a;
  	cin>>b;
  	int x[4],y[4];
  	x[0]=a[0]-48;
  	x[1]=a[1]-48;
  	x[2]=a[3]-48;
  	x[3]=a[4]-48;
  	y[0]=b[0]-48;
  	y[1]=b[1]-48;
  	y[2]=b[3]-48;
  	y[3]=b[4]-48;
  	
  	int k,l,m;
  	k=(x[0]*10+x[1])*60+(x[2]*10+x[3]);
  	l=(y[0]*10+y[1])*60+(y[2]*10+y[3]);
  	m=k+(l-k)/2;
  	int ans[4];
  	int temp=(x[0]*10 + x[1])+(y[0]*10+y[1]-x[0]*10 - x[1])/2;
  	 ans[1]=temp%10;
  	 ans[0]=(temp/10)%10;
  	int q=m-temp*60;
  	ans[2]=q%10;
  	ans[3]=(q/10)%10;
  	if(ans[3]>=6 && ans[2]>=0)
  	{
  		int temp2=ans[3]/6;
  		ans[1]=(ans[1]+temp2)%10;
  		ans[0]=ans[0]+(temp%10+temp2)/10;
  		ans[3]=ans[3]-6*temp2;
  		ans[2]=ans[2];
  	}
  	cout<<ans[0]<<ans[1]<<":"<<ans[3]<<ans[2];
  }
  