#include<stdio.h>



long calc(long n)
{long nr=0;

    while(n/5!=0)
	{
     n/=5;
	 nr+=n;
	}

 return nr;
}



int main()
{
	long p,c,a,b;
	FILE *f;
	
	
   
	
    f=fopen("fact.in","r");
	fscanf(f,"%li",&p);
	a=0;
	b=5*p;
    
	
	while(a<=b)
	{
		c=(a+b)/2;
		if(calc(c)>p) b=c-1;
		else if(calc(c)<p) a=c+1;
		else if(calc(c)==p) break;
		
	}


  f=fopen("fact.out","w");
  if(calc(c)==p) 
  {
	  while(calc(c)==p) c--;
	  fprintf(f,"%li",c+1);
  }
  //else fprintf(f,"%s","-1");


	return 0;
}