readLine()!
let s = readLine()!.map{String($0)}
let As: Set<String> = ["A","G","C","T"]
var c = 0
for i in s{
    if As.contains(i){
        c += 1
    }
}
var k = c, cnt = 0
c = c-1
while c >= 0 {
    if !As.contains(s[c]){
        while !As.contains(s[k]){
            k += 1
            cnt += 1
        }
        k += 1
        cnt += 1
    }
    c -= 1
    cnt += 1
}
print(cnt)