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

int log5(int a){
 int count=0;
 while(a/5!=0)
 {
     count++;
     a=a/5;
 }
 return count;
}

int main(int argc,char** argv){

  FILE *fp;
  int p;
  int val;
  int lsum;

  if((fp=fopen("fact.in","r"))==NULL){
     perror("fact.in :");
     return 1;
  }
  
  fscanf(fp,"%d",&p);
  fclose(fp);
  
  
  if((fp=fopen("fact.out","w"))==NULL){
     perror("fact.out :");
     return 1;
  }
  
  if(p==0)
    fprintf(fp,"%d",1);
  else{
    val=log5(p); 
    lsum=(val*(val+1))/2;
    fprintf(fp,"%d",5*(p-lsum));
  }

  fclose(fp);
  return 0;

}