#include <fstream>
#define Capat 100000000000000000/2
using namespace std;
ifstream f("fact.in");
ofstream g("fact.out");
long long n;
long long numar(long long y){
    long long s,p=5;
    long long t = y;
    while(y != 0){
        s += t/p;
        y = t/p;
        p *= 5;
    }
    return s;
}
long long caut(long long x){
    long long left = 0 , right = Capat;
    long long mij;
    while(left < right){
        mij = (left + right)/2;
        if(numar(mij)==x)
            return mij;
        else
        if(numar(mij) < x)
            left = mij + 1;
        else
        if(numar(mij) > x)
            right = mij - 1;
    }
}
int main()
{
    f >> n;
    if(n==0) g<<1;
    else{
    for(long long i = caut(n); i >= 0; i--){
        if(numar(i) != numar(caut(n))){
            g << i+1;
            return 0;
        }
    }
    g<<-1;}
    return 0;
}