import bisect

n = int(input())
s = input()
idx = []
c = 0
for i in range(n):
    if s[i] in ["A","C","G","T"]:
        c += 1
        idx.append(i)

if c == 0:
    print(0)
    exit()

c -= 1
if idx[0] >= c:
    print(idx[-1] + 1)
else:
    print(idx[-1] - idx[0] + 1)