1 #include<cstdio>
 2 #include<cstring>
 3 #include<cmath>
 4 #include<algorithm>
 5 #include<iostream>
 6 using namespace std;
 7 int f(int n)
 8 {
 9     if( n==1) 
10         return 0;
11     else
12         if(n==2)
13             return 1;
14         else
15             return f(n-1)+f(n-2); 
16 }
17 int main()
18 {
19     int n;
20     scanf("%d",&n);
21     printf("%d",f(n));
22     return 0;
23 }