package main

import . "fmt"

func main() {
	var a int
	Scan(&a)
	for x := 2; x <= 1000; x++ {
		v := Sprintf("%d%d", a/x%x, a%x)
		if a<x*x && v == "17" {
			Println(x)
			return
		}
	}
	Println(-1)
}