#include <stdio.h>

int n,nr;

int gen()
 {
  if (n%2==0) nr=10*(n/2);
	 else nr=10*(n/2)+5;
  return 1;
  }

int main()
{
 FILE *fin,*fout;

 fin=fopen("fact.in","r");
 fscanf(fin,"%d",&n);
 gen();
 fclose(fin);
 fout=fopen("fact.out","w");
 fprintf(fout,"%d",nr);
 fclose(fout);

}