RT 硬是没看出来为啥

#include<bits/stdc++.h>
#define int long long
const int maxn=1e6+5;
using namespace std;
inline int read(){
	   int x=0,f=1;char ch=getchar();
	   while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
	   while(ch>='0'&&ch<='9'){x*=10,x+=ch-'0';ch=getchar();}
	   return x*f;
}
int n,a,b,ans;
int c[105][105];
signed main(){
	
	n=read(),a=read(),b=read();
	int x=n+max(a,b);
	c[0][0]=1;
	c[1][0]=c[1][1]=1;
	for(int i=2;i<=x;i++){
		c[i][0]=1;c[i][i]=1;
		for(int j=1;j<=i;j++){
			c[i][j]=c[i-1][j-1]+c[i-1][j];
		}
	}
	for(int i=0;i<=a;i++){
		for(int j=0;j<=b;j++){
			ans+=c[n+i-1][n-1]*c[n+j-1][n-1];
		}
	}
	cout<<ans;
        return 0;
}