func main()
	var n: int :: cui@inputInt()
	var s: []char :: cui@input()
	
	var lis: list<char> :: #list<char>
	
	var cnt: int :: 0
	for i(0, ^s - 1)
		var c: char :: s[i]
		do lis.add(c)
		switch(c)
		case 'A', 'G', 'C', 'T'
			do cnt :+ 1
		end switch
	end for
	
	do lis.head()
	do lis.moveOffset(cnt - 1)
	
	var ans: int :: 0
	while(cnt <> 0)
		do ans :+ 1
		var c: char :: lis.get()
		switch(c)
		case 'A', 'G', 'C', 'T'
			do cnt :- 1
			if(cnt = 0)
				do lis.del()
			else
				do lis.prev()
				do lis.delNext()
			end if
		default
			do lis.del()
		end switch
	end while
	do cui@print("\{ans}\n")
end func