#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef vector<vector<long long>> VVL;
typedef pair<int,int> Pair;
typedef tuple<int,int,int> tpl;

#define ALL(a)  (a).begin(),(a).end()
#define SORT(c) sort((c).begin(),(c).end())
#define REVERSE(c) reverse((c).begin(),(c).end())
#define EXIST(m,v) (m).find((v)) != (m).end()
#define LB(a,x) lower_bound((a).begin(), (a).end(), x) - (a).begin()
#define UB(a,x) upper_bound((a).begin(), (a).end(), x) - (a).begin()

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define RFOR(i,a,b) for(int i=(a)-1;i>=(b);--i)
#define RREP(i,n) RFOR(i,n,0)

#define en "\n"

constexpr double EPS = 1e-9;
constexpr double PI  = 3.1415926535897932;
constexpr int INF = 2147483647;
constexpr long long LINF = 1LL<<60;
constexpr long long MOD = 1000000007; // 998244353;

template<class T> inline bool chmax(T& a, T b){if(a<b){a=b;return true;}return false;}
template<class T> inline bool chmin(T& a, T b){if(a>b){a=b;return true;}return false;}

struct modint {
    long long x;
    modint(long long x=0):x((x%MOD+MOD)%MOD){}
    long long val(){
        return x;
    }
    modint operator-() const { return modint(-x);}
    modint& operator+=(const modint a) {
        if ((x += a.x) >= MOD) x -= MOD;
        return *this;
    }
    modint& operator-=(const modint a) {
        if ((x += MOD-a.x) >= MOD) x -= MOD;
    	return *this;
    }
    modint& operator*=(const modint a) {
    	(x *= a.x) %= MOD;
        return *this;
    }
    modint operator+(const modint a) const {
        modint res(*this);
        return res+=a;
    }
    modint operator-(const modint a) const {
        modint res(*this);
        return res-=a;
    }
    modint operator*(const modint a) const {
        modint res(*this);
        return res*=a;
    }
    modint pow(long long t) const {
        if (!t) return 1;
        modint a = pow(t>>1);
        a *= a;
    	if (t&1) a *= *this;
        return a;
    }

	// must be gcd(x,MOD)==1
    modint inv() const {
        return pow(MOD-2);
    }

    modint& operator/=(const modint a) {
    	return (*this) *= a.inv();
    }
    modint operator/(const modint a) const {
        modint res(*this);
        return res/=a;
    }
};
using mint = modint;

struct Factorial{

    vector<mint> fact, ifact;

    Factorial(int N): fact(N+1), ifact(N+1) {
        assert(N < MOD);

        fact[0] = 1;
        for(int i=0; i<N; i++) fact[i+1] = fact[i] * (i+1);

        ifact[N] = fact[N].inv();
        for(int i=N; i>0; i--) ifact[i-1] = ifact[i] * i;
    }

    mint C(int n, int k){
        if (k < 0 || k > n) return 0;
        return fact[n]*ifact[k]*ifact[n-k];
    }

    mint P(int n, int k){
        if (k < 0 || k > n) return 0;
        return fact[n]*ifact[n-k];
    }

    mint inv(int n){
        assert(n>0);
        return fact[n-1]*ifact[n];
    }
};

mint f(ll n, ll r, mint nCr){
    mint ret = nCr;
    ret *= (n+1-r)%MOD;
    ret /= (n+1)%MOD;
    return ret;
}

mint g(ll n, ll r){
    mint bunbo(1),bunsi(1);
    ll mn = min(r,n-r);
    REP(i,mn){
        bunbo *= i+1;
        bunsi *= n-i;
    }
    return bunsi/bunbo;
}

void Main(){
    ll N,M; cin >> N >> M;
    mint nCr=g(N+M-1,M-1),ans;

    REP(i,N-1){
        ll a; cin >> a;
        nCr = f(N+M-2-i,M-1,nCr);
        ans += (nCr-1)*a + a;
    }
    cout << ans.x << en;
    return;
}

int main(void){
    cin.tie(0);cout.tie(0);ios_base::sync_with_stdio(0);cout<<fixed<<setprecision(15);
    int t=1; //cin>>t;
    while(t--) Main();
    return 0;
}