题解 | 数楼梯

import sys
def main():
    n=int(sys.stdin.readline())
    a=1
    b=2
    if n==1:
        print(a)
        return
    elif n==2:
        print(b)
        return
    else:
        for i in range(3,n+1):
            c=(a+b)%998244353
            a=b
            b=c
        print(b)
if __name__ =="__main__":
    main()