import sys
input = sys.stdin.readline
MOD = 998244353
M, N = map(int, input().split())
X = list(map(int, input().split()))
inv = pow(6, MOD-2, MOD)
ans = 0
bk = 1
for x in X:
    n = x-bk
    ans += n*(n+1)%MOD*(2*n+1)%MOD*inv%MOD
    ans %= MOD
    bk = x+1
n = M+1-bk
ans += n*(n+1)%MOD*(2*n+1)%MOD*inv%MOD
ans %= MOD
print(ans)