var a:real;
    n,b,c,d:longint;
begin
read(n);
b:=n mod 10;
c:=n div 10 mod 10;
d:=n div 100;
a:=(d+c)/(c+b);
write(a:0:2);
end.
/**************************************************************
	Problem: 1608
	User: admin
	Language: Pascal
	Result: Wrong Answer
****************************************************************/