m, n = map(int, input().split())

x = list(map(int, input().split())) + [m+1]

def add(l, r):
    x = r - l
    return x*(x+1)*(2*x+1)//6

miss = 0
ans = 0

for v in x:
    ans += add(miss+1, v)
    ans %= 998244353
    miss = v

print(ans)