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

int sqr (int x) {
   int n = 0;

   while(x>=5) {
     x = x / 5;
     n++ ;
   }
   return n;
}

int power (int x) {
   int n = 1;
 
   while(x) {
	n = n * 5;
	x--;
	}

   return n;
}

int main(void) {
   int nr;
   FILE *f,*g;
   f = fopen("factorial.in", "r");
   g = fopen("factorial.out", "w");
   fscanf(f, "%d", &nr);
   int i,j, v[10000];

   for(i = 1; i < nr; i++) v[i] = i;
   for(i = 1; i < nr ; i++ ){
     if(sqr(i) == power(sqr(i)))
        for(j = i+1; j < sqr(i); j++) v[j]=0;
   }
   j = nr;
   while(!v[j]) j--;
   fprintf(g, "%d\n", j*5+5);
   return 0;

}