n = int(input())
s = input()
c = sum(s.count(e) for e in "AGCT") - 1
l = max(0, min(s.find(e) for e in "AGCT"))
r = max(0, max(s.rfind(e) for e in "AGCT"))
if c == 0:
    print(0)
else:
    # print(c, l, r)
    print(max(c, l, r) - min(c, l, r) + 1)