recursion memoization

#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <stack>
#include <vector>
#include <math.h>
#include <queue>
using namespace std;
//    5C3 = 4C2 + 4C3

int dy[21][21];

int DFS(int n, int r){
	if(dy[n][r]>0) return dy[n][r];
	if(r==0 || n==r) return 1;
	else{
		return dy[n][r] = DFS(n-1, r-1) + DFS(n-1, r);
	}
}
int main() {
	//freopen("input.txt.txt","rt",stdin);
	int n, r;
	scanf("%d %d", &n, &r);
	printf("%d", DFS(n,r));
	return 0;	
}