M,N = map(int,input().split())
X = [0]+list(map(int,input().split()))+[M+1]
MOD = 998244353
ans = 0
for i in range(1,N+2):
    x0 = X[i-1]
    x1 = X[i]
    n = x1-x0-1
    ans = (ans+(n*(n+1)*(2*n+1))//6)%MOD
print(ans)