为啥测试用例2错了呀？？？

#include <iostream>
#include <cstring>
#include <queue>

#define x first
#define y second

using namespace std;

typedef pair<int, int> pii;

const int N = 410;

int n, m;
int dx[8] = {-2, -1, 1, 2, -2, -1, 1, 2}, dy[8] = {-1, -2, -2, -1, 1, 2, 2, 1};
int g[N][N];
bool vis[N][N];


bool check(int a, int b, int n, int m) {
    if(a < 1 || a > n || b < 1 || b > m) return true;
    return false;
}

void bfs(int sx, int sy) {
    queue<pii> q;
    q.push({sx, sy});
    vis[sx][sy] = true;
    while(!q.empty()) {
        auto t = q.front();
        q.pop();
        for(int i = 0; i < 8; i ++) {
            int a = t.x + dx[i], b = t.y + dy[i];
            if(check(a, b, n, m) || vis[a][b]) continue;
            g[a][b] = g[t.x][t.y] + 1;
            q.push({a, b});
            vis[a][b] = true;
        }
    }
}

int main() {
    cin >> n >> m;
    int sx, sy;
    cin >> sx >> sy;
    bfs(sx, sy);
    for(int i = 1; i <= n; i ++)
        for(int j = 1; j <= m; j ++)
            if(!g[i][j] && i != sx && j != sy) 
                g[i][j] = -1;
    for(int i = 1; i <= n; i ++) {
        for(int j = 1; j <= m; j ++) printf("%-5d", g[i][j]);
        puts("");
    }
    return 0;
}