> 문제 <

https://www.acmicpc.net/problem/2193

> 아이디어 <

> 코드 <

n = int(input())
dp = [[0] * 2 for _ in range(n+1)]

dp[1][1] = 1

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

print(sum(dp[n]))