#include "bits/stdc++.h"
#include <gmp.h>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> veci;
typedef vector<ll> vecll;
typedef vector<string> vecs;
template<class T,class U> using Hash=unordered_map<T,U>;

#define REP(i, a, n) for(ll i = a; i < n; i++)
#define RREP(i, a, n) for(ll i = n-1; i >= a; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define MD 1000000007

template<class T> T read(){T a;cin >> a;return a;}
template<class T> void read(T& a){cin >> a;}
template<class T, class ...Args> void read(T& a, Args&... args){cin >> a; read(args...);} 
template<class T> void rarr(T a, int n){for(int i = 0; i < n; i++) {cin >> a[i];}}
template<char c = ' ',class T> void write(T a){cout << setprecision(12) << a << endl;}
template<char c = ' ',class T,class ...Args> void write(T a, Args... args){cout << setprecision(12) << a << c; write<c>(args...);}
template<class T> void warr(vector<T> a, const char* c = " "){cout << a[0];for(int i = 1; i < (int)a.size(); i++)cout << c << a[i];cout << endl;;}
template<class T> void warr(T a, int n, const char* c = " "){cout << a[0];for(int i = 1; i < n; i++)cout << c << a[i];cout << endl;}
void split(string s, string delim, veci& result){result.clear();string::size_type pos = 0;while(pos != string::npos){string::size_type p = s.find(delim, pos);if(p == string::npos){result.push_back(atoi(s.substr(pos).data()));break;}else {result.push_back(atoi(s.substr(pos, p - pos).data()));}pos = p + delim.size();}}
void split(string s, string delim, vecs& result){result.clear();string::size_type pos = 0;while(pos != string::npos){string::size_type p = s.find(delim, pos);if(p == string::npos){result.push_back(s.substr(pos));break;}else {result.push_back(s.substr(pos, p - pos));}pos = p + delim.size();}}
ll gcd(ll a, ll b){while(true){ll k = a % b;if(k == 0)return b;a = b;b = k;}}
ll comb(ll n, ll m){ll p=1;m=min(m,n-m);for(ll i=1;i<=m;i++){p*=n-i+1;p/=i;}return p;}

ull power( const ull& a, ull k, const ull& n )
{
  if ( a == 0 || n == 0 ) return( 0 );
  if ( k == 0 ) return( 1 % n );

  ull currentMod = a % n;
  ull currentValue = ( ( k & 1 ) > 0 ) ? currentMod : 1;

  for ( k >>= 1 ; k > 0 ; k >>= 1 ) {
    currentMod = ( currentMod * currentMod ) % n;
    if ( ( k & 1 ) > 0 )
      currentValue = ( currentValue * currentMod ) % n;
  }

  return( currentValue );
}

inline ll mult(ll a,ll b){ return ((a%MD)*(b%MD))%MD; }
inline ll divi(ll a, ll b){ return (((a%MD)*power(b,MD-2,MD))%MD); }
inline ll plu(ll a, ll b){ return (a+b)%MD; }
inline ll minu(ll a, ll b){ return (a-b+MD)%MD; }

int main(void)
{
    
    ll frac[10100]={1},inv_frac[10100]={1};
    REP(i,1,10100)frac[i]=(frac[i-1]*i)%MD;
    REP(i,1,10100)inv_frac[i]=power(frac[i],MD-2,MD);
    ll n,k;
    read(n,k);
    ll br[10100]={n,divi(mult(n,n+1),2)};
    REP(i,2,k+1){
        rep(j,i){
            ll c=mult(frac[i+1],mult(inv_frac[i-j+1],inv_frac[j]));
            //write(i+1,j,c);
            if((i+j)%2==0)br[i]=plu(br[i],mult(c,br[j]));
            else br[i]=minu(br[i],mult(c,br[j]));
        }
        br[i]=minu(br[i],power(n,i+1,MD));
        br[i]=divi(br[i],minu(0,i+1));
    }
    write(br[k]);
	return 0;
}