#include "stdio.h" 

int main() 
{ 
int a,b,c,d; 
FILE *F; 
FILE *G; 
F = fopen("fact.in","r"); 
G = fopen("fact.out","w"); 
fscanf(F,"%d",&a); 
/*
b = 0;
c = 0;
if (( b == 0) && (c == 0)){
		d = - 1;
		return 0;
}


while (a % 5 == 0)
{
	a = a / 5;
	b++;
}

while (a % 2 == 0)
{
	a = a / 2;
	c++;
}

if (b > c){
	d = c;
}else{
	d = b;
}
*/
d = (a/2)* 5;

fprintf(G,"%d",d); 
fclose(G); 
fclose(F); 
return 0; 
}