#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint1000000007;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000
int main(){
	
	long long N,M;
	cin>>N>>M;
	
	vector<long long> A(N,0LL);
	rep(i,N-1)cin>>A[i+1];
	
	mint cur = 1;
	int pos = -100;
	rep(i,N){
		if((N+M-1-i)%1000000007==0){
			pos = i;
			break;
		}
		cur /= N-i;
		
			
		cur *= N+M-1-i;
	}
	mint ans = 0;
	for(int i=1;i<N;i++){
		/*
		cout<<i<<','<<N+M-i<<endl;
		if(N+M-i==1000000007){
			cout<<pos<<','<<i<<endl;
		}
		*/
		if(pos!=-100){
			if(pos+1!=i){
				cur /= N+M-i;
				cur *= N+1-i;
				pos = -100;
			}
			else continue;
		}
		else{
			cur /= N+M-i;
			cur *= N+1-i;
		}
		ans += cur * A[i];
		//ans -= (M-1)*A[i];
	}
	
	cout<<ans.val()<<endl;
	
    return 0;
}