#include <fstream.h>
int main(){
int m,n,k,i,j;
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;

     for (j=i+1;j<=m;j++)
     p2=p2*j;

     for (j=1;j<=m-i;j++)
     p2=p2/j;
     p=p+p2;
     }
     fout<<p;
     fout.close();
     return 0;
     }