package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Buffer(make([]byte, 500000+2), 500000+2)
	sc.Scan()
	sc.Scan()
	s := sc.Text()
	tt := []byte(s)

	n := 0
	for i, t := range tt {
		if t == 'A' || t == 'G' || t == 'C' || t == 'T' {
			n = 1 + i
		}
	}
	fmt.Println(n)
}