#include using namespace std;
long long fib(int n){
long long a = 0;
long long b = 1;
long long c = a + b;
while(n > 2){
a = b;
b = c;
c = a + b;
--n;
}
return c;
}
int main() { int n; cin >> n; long long sum = fib(n); cout << sum ;
return 0;
}