import sys
if sys.platform =='ios':
	import clipboard
	a=clipboard.get()
	a = a.split('\n')
	text = '\n'.join(a)
	with open('input_file.txt','w') as f:
		f.write(text)
	sys.stdin = open('input_file.txt')
	
n = int(input())
s = input()
cnt = 0
A = ["A","G","C","T"]
for i in range(n-1,-1,-1):
	if s[i] in A:
		print(i+1)
		exit()
print(0)