import sys

def main():
    j = sys.stdin.read()
    if j[0] != "1":
        sys.stdout.write(str(-1) + "\n")
        return
    n = 0
    for v in j[1:]:
        if v == "3":
            n += 1
        elif v != "3":
            sys.stdout.write(str(-1) + "\n")
            return
    sys.stdout.write(str(n) + "\n")