#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
using lint = long long;
using ulint = unsigned long long;
#define endl '\n'
int const INF = 1<<30;
lint const INF64 = 1LL<<61;
lint const mod = 1e9+7;
//long const mod = 998244353;
lint ceilDiv(lint x, lint y){if(x >= 0){return (x+y-1)/y;}else{return x/y;}}
lint floorDiv(lint x, lint y){if(x >= 0){return x/y;}else{return (x-y+1)/y;}}
lint Sqrt(lint x){lint upper = 1e9;lint lower = 0;while(upper - lower > 0){lint mid = (1+upper + lower)/2;if(mid * mid > x){upper = mid-1;}else{lower = mid;}}return upper;}
lint gcd(lint a,lint b){if(a<b)swap(a,b);if(a%b==0)return b;else return gcd(b,a%b);}
lint lcm(lint a,lint b){return (a / gcd(a,b)) * b;}
lint chmin(vector<lint>&v){lint ans = INF64;for(lint i:v){ans = min(ans, i);}return ans;}
lint chmax(vector<lint>&v){lint ans = -INF64;for(lint i:v){ans = max(ans, i);}return ans;}
double dist(double x1, double y1, double x2, double y2){return sqrt(pow(x1-x2, 2) + pow(y1-y2,2));}
string toString(lint n){string ans = "";if(n == 0){ans += "0";}else{while(n > 0){int a = n%10;char b = '0' + a;string c = "";c += b;n /= 10;ans = c + ans;}}return ans;}
vector<lint>prime;void makePrime(lint n){prime.push_back(2);for(lint i=3;i<=n;i+=2){bool chk = true;for(lint j=0;j<prime.size() && prime[j]*prime[j] <= i;j++){if(i % prime[j]==0){chk=false;break;}}if(chk)prime.push_back(i);}}
struct edge{
    int to;
};
using graph = vector<vector<edge>>;
lint kai[20000001];
bool firstCallnCr = true;

lint ncrmodp(lint n,lint r,lint p){
    if(firstCallnCr){
        kai[0] = 1;
        for(int i=1;i<=mod;i++){
            kai[i] = kai[i-1] * i;
            kai[i] %= mod;
        }
        firstCallnCr = false;
    }
    if(n >= mod)return 0;
    if(n<0)return 0;
    if(n < r)return 0;
    if(n==0)return 1;
    lint ans = kai[n];
    lint tmp = (kai[r] * kai[n-r]) % p;

    for(lint i=1;i<=p-2;i*=2){
        if(i & p-2){
            ans *= tmp;
            ans %= p;
        }
        tmp *= tmp;
        tmp %= p;
    }
    return ans;
}
lint f(lint n, lint m){
    lint tmp = 1;
    tmp *= m+2;
    tmp *= ncrmodp(n+m+2, n, mod);
    tmp -= n+1;
    tmp += mod;
    tmp %= mod;
    tmp *= inv_mod(n+1, mod);
    return tmp;
}

int main(){
    lint n,m;
    cin >> n >> m;
    lint a[n+1];
    for(int i=2;i<=n;i++)cin >> a[i];
    lint ans = 0;
    for(int i=2;i<=n;i++){
        ans += a[i] * f(n-i, m-2);
        ans %= mod;
    }
    for(int i=2;i<=n;i++){
        ans += a[i];
        ans %= mod;
    }
    cout << ans << endl;
}