n = int(input())
s = list(input())
counter = 0
start = s.count('A') + s.count('G') + s.count('C') + s.count('T')
checklist = ['A','G','C','T']
f = [0]*n
for sn in range(n):
    if s[sn] in checklist:
        f[sn] = 1
if start == 0: print(0)
else:
    goof = [i for i in range(n) if f[i] == 1]
    left = goof[0]
    right = goof[-1]
    r_nagasa = len([i for i in range(len(goof)) if start <= i])
    l_nagasa = len(goof) - r_nagasa
    print(right-start+l_nagasa+1)