#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;
	
	rep(i,N){
		cur /= N-i;
		cur *= N+M-1-i;
	}
	mint ans = 0;
	for(int i=1;i<N;i++){
		cur /= N+M-i;
		cur *= N+1-i;
		//cout<<cur.val()<<endl;
		ans += cur * A[i];
		//ans -= (M-1)*A[i];
	}
	
	cout<<ans.val()<<endl;
	
    return 0;
}