def II() -> int : return int(input())
def MI() -> int : return map(int, input().split())
def TI() -> tuple[int] : return tuple(MI())
def LI() -> list[int] : return list(MI())
class fenwick_tree():
    n=1
    data=[0 for i in range(n)]
    def __init__(self,N):
        self.n=N
        self.data=[0 for i in range(N)]
    def add(self,p,x):
        assert 0<=p<self.n,"0<=p<n,p={0},n={1}".format(p,self.n)
        p+=1
        while(p<=self.n):
            self.data[p-1]+=x
            p+=p& -p
    def sum(self,l,r):
        assert (0<=l and l<=r and r<=self.n),"0<=l<=r<=n,l={0},r={1},n={2}".format(l,r,self.n)
        return self.sum0(r)-self.sum0(l)
    def sum0(self,r):
        s=0
        while(r>0):
            s+=self.data[r-1]
            r-=r&-r
        return s

mod = 998244353
m,n = MI()
x = LI()
x = [0] + x + [m+1]
# print(x)

ans = 0
for i in range(1,n+2):
    d = x[i]-x[i-1]-1
    # print(d)
    ans += pow(6,-1,mod)*d*(d+1)%mod*(2*d+1)%mod

print(ans%mod)