#include<bits/stdc++.h>

using namespace std;

int main(){
    int A;
    cin>>A;
    for(int X=2;X<=100;X++){
	int a1=A%X;
	int a2=(A/X)%10;
	int rest=(A/X)/X;
	if(rest==0&&a2==1&&a1==7){
	    cout<<X<<endl;
	    return 0;
	}
    }
    cout<<-1<<endl;
    return 0;

}