import sys
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
INF = 2 ** 63 - 1
mod = 998244353

ans = 0

n = ii()

s = input()

for i in range(n):
    if s[i] in 'AGCT':
        ans = i + 1
print(ans)