#include<cstdio>
#include<cstring>
#include<cmath>
#include<cstdlib>
#include<ctime>
#include<iostream>
#include<algorithm>
#include<set>
#include<map>
#include<queue>
#include<bitset>
#include<vector>
using namespace std;
namespace mine
{
	#define double long double
	typedef long long ll;
	#define pr pair<int,int>
	#define FR first
	#define SE second
	#define MP make_pair
	#define PB push_back
	#define vc vector
	#define mid (l+r)/2
	#define lc 2*x
	#define rc 2*x+1
	void chmax(int &x,const ll y) {x=(x>y?x:y);}
	void chmin(int &x,const ll y) {x=(x<y?x:y);}
	ll qread()
	{
		ll ans=0,f=1;char c=getchar();
		while(c<'0' or c>'9') {if(c=='-')f=-1;c=getchar();}
		while('0'<=c and c<='9') ans=ans*10+c-'0',c=getchar();
		return ans*f;
	}
	void write(ll num)
	{
		if(num<0) putchar('-'),num=-num;
		if(num>=10) write(num/10);
		putchar('0'+num%10);
	}
	void write1(ll num){write(num);putchar(' ');}
	void write2(ll num){write(num);putchar('\n');}
	const int INF=0x3f3f3f3f;
	const int MOD=1e9+7;
	void add(int &a,ll b){a+=b;if(a>=MOD)a-=MOD;if(a<=-MOD)a+=MOD;}
	ll qpower(ll x,ll e)
	{
		ll ans=1;
		while(e)
		{
			if(e&1) ans=ans*x%MOD;
			x=x*x%MOD;e>>=1;
		}
		return ans;
	}
	ll invm(ll x){return qpower(x,MOD-2);}
	const int N=1e6+10;

	ll A[N];
	void main()
	{
	    int n=qread(),K=qread();
	    A[0]=1;ll pp=1;
	    for(int i=1;i<=n;i++)
	    {
	        int a=qread();pp=pp*a%MOD;
			for(int j=n;j>=1;j--) A[j]=(A[j]*a-A[j-1])%MOD;A[0]=A[0]*a%MOD;
	    }
		ll ans=0;
		for(int i=0,down=1;i<=n and i<=K;i++)
		{
			(ans+=A[i]*invm(qpower(n,i))%MOD*down%MOD )%=MOD;
			down=(ll)down*(K-(i+1)+1)%MOD;
		}
		write((pp-ans+MOD)%MOD);
	}
};
int main()
{
	srand(time(0));
	mine::main();
}