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