inf=10**18
dxy=[(0,1), (1,0), (-1, 0), (0, -1)]
mod=10**9+7
MOD=998244353
def main():
    N = int(input())
    S = input()
    cnt = 0
    keys = ["A", "G", "C", "T"]
    for i,n in enumerate(S[::-1]):
        if n in keys:
            print(N-i)
            return
    print(0)
main()