#include<fstream>
#include<cmath>
using namespace std;

int calc(long long);

int main(){
    ifstream inFile("fact.in");
    ofstream outFile;
    outFile.open("fact.out");
    long long p;
    inFile>>p;
    int i;
    for(i=1;calc(i)<p;i++);
    outFile<<i;


}


int calc(long long n){   //Calculam exponentul lui 5 in n!
    int expFive=0; //expTwo=0;
    for(int i=1;n/pow(5,i)>0;i++) expFive+=n/pow(5,i);
  /*
    for(int i=1;n/pow(2,i)>0;i++) expTwo+=n/pow(2,i);
    int k;
    k=(expTwo>expFive)?expFive:expTwo;
  */
    return expFive;
}