#include<cstdio>
using namespace std;

int main()
{
    FILE *f=fopen("fact.in","r"),*g=fopen("fact.out","w");
    int x,facto=1,c=0,i,j;
    bool gasit=false;
    fscanf(f,"%i",&x);
    if(x<0)
    {
        fprintf(g,"-1");
    }
    else
    {
        if(x==0)
        {
            fprintf(g,"1");
        }
        else
        {
            for(i=1;i<x*5;i*=5)
            {
                c++;
            }
            c--;
            fprintf(g,"%i",x*5-c*(c-1)*5/2);
        }

    }

    fclose(f);
    fclose(g);
    return 0;
}