use proconio::input;
use std::collections::HashMap;
use std::collections::BTreeSet;
use std::ops::ControlFlow::Break;
fn main(){
    input!{
        n:isize,
    }
    let mut ans=-1;
    for i in (8..100){
        let mut c=0;
        let mut d=1;
        let mut f=n;
        while f>0{
            if f%i>=10{
            	c=100;
            	break;
            }
            c+=d*(f%i);f/=i;d*=10;
        }
        if c==17{
            ans=i;break;
        }
    }
    print!("{}",ans);
}