n = int(input())
s = input()

A = [i for i, x in enumerate(s) if x == "A"]
G = [i for i, x in enumerate(s) if x == "G"]
C = [i for i, x in enumerate(s) if x == "C"]
T = [i for i, x in enumerate(s) if x == "T"]

AGCT = A + G + C + T
AGCT.sort()
if len(AGCT) == 0:
    print(0)
else:
    print(AGCT[-1] + 1)