mystery wrong

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int n,m,a[2005][2005],num;
int head[500005],to[500005],next[500005];
bool used[500005],match[500005];
int ans;
int dx[9]={0,2,2,1,1,-1,-1,-2,-2};
int dy[9]={0,1,-1,2,-2,2,-2,1,-1};
int id(int x,int y)
{
    return (x-1)*m+y;
}
void make_way(int u,int v)
{
    to[++num]=v;
    next[num]=head[u];
    head[u]=num;
}
int dfs(int u)
{
    int t;
    for(int edge=head[u];edge;edge=next[edge])
    {
        int v=to[edge];
        if(used[v]) continue;
        t=match[v];
        match[v]=u;
        used[v]=1;
        if(!t||dfs(t)) return 1;
        match[v]=t;
    }
    return 0;
}
int main()
{
    scanf("%d %d",&n,&m);
    for(int i=1;i<=n;i++)
        for(int j=1;j<=m;j++)
            scanf("%d",&a[i][j]);
    for(int i=1;i<=n;i++)
        for(int j=1;j<=m;j++)
        if(!a[i][j]&&(i+j)%2==1)
        {
            for(int dir=1;dir<=8;dir++)
            if(i+dx[dir]>=1&&i+dx[dir]<=n&&j+dy[dir]>=1&&j+dy[dir]<=m&&!a[i+dx[dir]][j+dy[dir]])
            {
                make_way(id(i,j),id(i+dx[dir],j+dy[dir]));
                cout<<id(i,j)<<' '<<id(i+dx[dir],j+dy[dir])<<endl;
            }
        }    
    int sum=0;    
    for(int i=1;i<=n;i++)
    {
        for(int j=1;j<=m;j++)
        if(!a[i][j])
        {
            sum++;
            if((i+j)%2==1)
            {
                memset(used,0,sizeof(used));
                if(dfs(id(i,j))) ans++;
            }
        }
    }
    cout<<match[6]<<endl;
    cout<<ans<<endl;
    printf("%d
",sum-ans);        
 }