#include <fstream.h>
int main(){
int m,n,k,i,j,l,m2;
long double p=0,p2;
ifstream fin("nunta.in");
ofstream fout("nunta.out");

fin>>n;
p=p+1;
for (i=1;i<=n/2;i++) {
     m=i+(n-i*2);
     p2=1;
     k=i;
     l=n-i*2;
     m2=m;
     m=m-1;
     while (m>=0){
     p2=p2*(m2-m);
     if (k>0) {p2=p2*k; k++;}
     if (l>0) {p2=p2*l; l++;}
     }
      p=p+p2;
     }
     fout<<p;
     fout.close();
     return 0;
     }