n=int(input())
if n==1:
    print(1)
    exit()
if n==2:
    print(2)
    exit()

dp=[0]*(n+1)
dp[1]=1
dp[2]=2
dp[3]=3
for i in range(4,n+1):
    dp[i]=dp[i-1]+dp[i-2]

print(dp[n])