import sys a = int(input()) if a % 8 == 0: print(a // 8) elif a % 8 == 6: print(a // 8 + 1) elif a % 8 == 2 and a >= 18: print(a // 8 + 1) elif a % 8 == 4 and a >= 12: print(a // 8 + 1) else: print(-1)