from collections import Counter
n = int(input())
s = list(input())

agctcnt = 0
agctset = set(["A", "G", "C", "T"])

agctmaxindex = -1

for index, it in enumerate(s):
  if it in agctset:
    agctcnt += 1
    agctmaxindex = index

print(agctmaxindex + 1)