#include <stdio.h>
#include <stdlib.h>

int main()
{
    FILE *pf;
    pf=fopen("fact.in","r");
    int p;
    fscanf(pf,"%d",&p);
    fclose(pf);
    pf=fopen("fact.out","w");
    double aux=p;
    int x=p;
    while (aux>=5)
    {
        aux=aux/5;
        x=x-floor(aux)+1;
    }
    fprintf(pf,"%d",x*5);
    fclose(pf);
    return 0;
}