using namespace std;
#include<fstream>
int main()

{

ifstream f("fact.in");
ofstream g("fact.out");

int p,a,b,i,div5=0,div2=0,nr0=0;
unsigned int n=1;
f>>p;
for(i=2; i<=100000000 && p>nr0; i++)
{
n++;
a=i;
b=i;
while(a%2==0)
{div2++;
a=a/2;}
while(b%5==0)
{div5++;
b=b/5;}
if(div2>=1&&div5>=1)
{if(div2<=div5)
nr0=div2;
else
nr0=div5;} 
}
g<<n;
}