#include<stdio.h>
int main(){
int n,a,t1=1,t=1;
scanf("%d",&a);
n=a-1;
int sum =1;
for(int i=1;i<=n/2;i++){
t1*=i;
t=1;
for(int j=n-i;j>n-i*2;j--){
t*=j;
}

t/=t1;

sum+=t;
}

printf("%d",sum);
return 0;
}