#include <fstream>
using namespace std;
ifstream in("fact.in");
ofstream out("fact.out");
int main()
{
    int f=0,nrz,z=1,p;
    in>>p;
    nrz=p;
   while(nrz>0)
   {
        nrz=nrz-z;
        z++;
        f++;
    }
    out<<f*5;
}