Lähetit

#include <iostream>

#define ll long long
#define ull unsigned long long

#define pii pair<int, int>
#define pll pair<long long, long long>

#define defmod 1000000007
using namespace std;

ll s[2020][2020] = {0};
ll d[2020][2020] = {0};
int main(){
	cin.sync_with_stdio(0);
	cin.tie(0);
	
	ll n, k; cin >> n >> k;
	
	s[0][0] = 1;
	for (int i = 1; i <= 2000; i++) {
		for (int j = 1; j <= i; j++) {
			s[i][j] = j*s[i-1][j]+s[i-1][j-1];
			s[i][j]%=defmod;
		}
	}
	for (int i = 0; i <= 2000; i++) {
		d[i][0] = 1;
		for (int j = 1; j <= i; j++) {
			d[i][j] = d[i-1][j-1]+d[i-1][j];
			d[i][j]%=defmod;
		}
	}

	ll re = 0;
	for(ll i = max(0LL, k-n); i <= min(k, n); i++){
		ll a = 0;
		for(ll j = 0; j <= (n+1)/2; j++){
			a+=((d[(n+1)/2][j])*s[j+n/2][n-i]);
			a%=defmod;
		}
		ll b = 0;
		for(ll j = 0; j <= n/2; j++){
			b+=(d[n/2][j])*(s[j+(n+1)/2][n-k+i]);
			b%=defmod;
		}

		re+=a*b;
		re%=defmod;
	}
	cout << re << endl;
	return 0;
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 2

Verdict: ACCEPTED

Test 7

Group: 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 3

Verdict: ACCEPTED

Test 12

Group: 3

Verdict: ACCEPTED

Test 13

Group: 3

Verdict: ACCEPTED

Test 14

Group: 3

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED