> #include <stdio.h>
> #include <string.h>
> 
> int parse(char str[])
> {
> 	int ans = ((str[0] - '0')*10 + (str[1] - '0'))*3600;
> 	ans += ((str[3] - '0')*10 + (str[4] - '0')) * 60;
> 	ans += (str[6] - '0')*10 + (str[7] - '0');
> 	return ans;
> }
> 
> int main()
> {
> 	char in[10];
> 	while(scanf("%s",in) != EOF&&in[0] != '-')
> 	{
> 		int t[6];
> 		t[1] = parse(in);
> 		for(int i= 2;i <= 5;i ++)
> 		{
> 			scanf("%s",in);
> 			t[i] = parse(in);
> 		}
> 		double tmp = (t[3] - t[1])*1.0/(t[4] - t[1])*(t[5] - t[2])*1.0/(t[3] - t[2]);
> 		double tt = (t[5] - tmp*t[4])/(1-tmp);
> 		int ttt = (int)(tt + 0.5);
> 		printf("%.2d:%.2d:%.2d\n",(ttt/3600)%24,(ttt%3600)/60,ttt%60);
> 	}
> 	return 0;
> }