# # # No.1941 CHECKER×CHECKER(1)
# # S=[input() for _ in range(3)]
# # ichimatu=["#.#",".#.","#.#"]
# # ichima2=[".#.","#.#",".#."]
# # if S==ichimatu or S==ichima2:
# #     print("Yes")
# # else:
# #     print("No")

# # No.1942 Leading zero
# N=int(input())
# for _ in range (N):
#     print(int(input()))

# No.1943 消えたAGCT(1)
AGCT=set(["A","G","C","T"])
n=int(input())
s=list(input())
ans=0
for i in range (n):
    if s[i] in AGCT:
        ans=i+1

print(ans)