#include <fstream>
int main()
{
    int P,n=0;
    std::ifstream nrzerouri ("fact.in");
    std::ofstream nrnat ("fact.out");
    nrzerouri>>P;
    int pfive = 25;
    int expfive = 1;
    while (P>0){
        n=n+5;
        P--;
        if (n%pfive==0){
            P-=expfive;
            expfive++;
            pfive*=5;
        }
    }
    if (n==0){
        n++;
    }
    if(P<0) nrnat<<-1;
    else nrnat<<n;
}