#include <stdio.h>


int main()
{
    FILE *f = fopen("fact.in", "r") ;
    FILE *g = fopen("fact.out", "w");
    
    if (f == NULL || g == NULL)
    {
          return -1;
    }
    
    int p, count = 1, x = 0,y = 0;
    
    fscanf(f, "%d", &p);
    
    while (count < p)
    {
          x++;
          count++;
          if (count < p) 
          {
              y++;
              count++;
          }
    }
    
    if (x == 0) fprintf(g, "%d", 1);
    else if (x == y) fprintf(g, "%d", count * 10 * x);
    else fprintf(g, "%d", (count-1) * 5);
    
    fclose(g);
    
    return 0;
}