#include
using namespace std;
int distinct_ele(char *out, int i, int n){
//if we have reached the last element
if(i==n){
return 1;
}
out[i]='a';
int distinct_a=distinct_ele(out,i+1,n);
int distinct_b=0; //let's assume can't put
if(i!=0 && out[i-1]!='b'){
out[i]='b';
distinct_b=distinct_ele(out,i+1,n);
}
return distinct_b+distinct_a;
}
int main(){
#ifndef ONLINE_JUDGE
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
int n;
char out[100];
cin>>n;
cout<<distinct_ele(out,0,n);
return 0;
}