#include<stdio.h>

FILE *fin=fopen("fact.in","r");
FILE *fout=fopen("fact.out","w");

int save,n,i,q,k;

int main(){
	
	fscanf(fin,"%d",&n);
	
	q=0;
	
	if(n==0) fprintf(fout,"1");
	
	else{
		
		for(i=1;i<=n;i++){
			
			if(i%5) q+=5;
			
			else{
				save=i;k=0;
				while(save%5==0){
					q+=5;
					save/=5;
					k++;
				}
				
				i+=k;
				
			}
			
		}
		
	}
	
	fprintf(fout,"%d",q);
	
	fclose(fin);
	fclose(fout);
	return 0;
	
}