mod = 998244353
N = 2*10**5
inverse = [1]*(N+1)

for i in range( 2, N + 1 ):
    inverse[i]=( ( -inverse[mod % i] * (mod//i) ) % mod )
inverse[0]=0

N,M = map(int,input().split())
A = [0] + list(map(int,input().split()))

res = 0
for j in range(M):
    for i in range(A[j],A[j+1]):
        res += A[j+1] * inverse[N-i]
        res %= mod

print(res)