import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n = ni()
s = input()[::-1]
def f(x):
    if s.find(x)==-1:
        return n
    else:
        return s.find(x)
print(n-min(f("C"),f("A"),f("G"),f("T")))