P = 998244353
nn = 1001001
fa = [1] * (nn+1)
fainv = [1] * (nn+1)
for i in range(nn):
    fa[i+1] = fa[i] * (i+1) % P
fainv[-1] = pow(fa[-1], P-2, P)
for i in range(nn)[::-1]:
    fainv[i] = fainv[i+1] * (i+1) % P

N, M = map(int, input().split())
A = [int(a) for a in input().split()]
B = [0] * N
i = 0
for a in A:
    while i < a:
        B[i] = a
        i += 1
ans = 0
for i in range(N):
    ans = (ans + fainv[N-i] % P * fa[N-i-1] % P * B[i]) % P
print(ans)