N = int(raw_input())
sugoroku = [1]
for i in range(0, N):
	sugoroku.append(0)
for i in range(0, N):
	sugoroku[i+1] += sugoroku[i]
	if(i+2 <= N):
		sugoroku[i+2] += sugoroku[i]
print sugoroku[N]