#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define _GLIBCXX_DEBUG
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
ll ILL=1167167167167167167;
const int INF=100000000;
const ll mod=1e9+7;
#define rep(i,a) for (ll i=0;i<a;i++)
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
void yneos(bool a){if(a) cout<<"YES\n"; else cout<<"NO\n";}

ll jyo(ll x,ll y,ll z){
  ll H=y; //ここから
       ll a=1,b=x,c=1;
       while(H>0){
         a*=2;
         if(H%a!=0){
           H-=a/2;
           c*=b;
           c%=z;
         }
        b*=b;
         b%=z;
      } //ここまで
 return c;
}

void solve();

//  rainy ~ 雨に打たれて ~
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int t=1;
	//cin>>t;
	rep(i,t) solve();
}

void solve(){
	ll N,M;
	cin>>N>>M;
	vector<ll> A(N);
	rep(i,N-1) cin>>A[i+1];
	for(int i=N-2;i>0;i--) A[i+1]-=A[i];
	ll R=1;
	ll B=1;
	ll C=1;
	rep(i,N-1){
		R*=(i+1);
		R%=mod;
		ll x=(N+M-1-i)%mod;
		if(x==0) C--;
		else B=(B*x)%mod;
	}
	R=jyo(R,mod-2,mod);
	ll ans=0;
	for(ll i=1;i<N;i++){
		//cout<<B<<" "<<jyo(R,mod-2,mod)<<"\n";
		ans+=(((B*R*C-1)%mod)*A[i])%mod;
		ans%=mod;
		ll x=(N+M-i)%mod;
		if(x==0) C++;
		else B=(B*jyo(x,mod-2,mod))%mod;
		R=(R*(N-i))%mod;
	}
	cout<<(ans%mod+mod)%mod<<"\n";
}
/*
(N+M-i)C(N-i) -1
*/