样例过了，但全 WA，求助。

#include<bits/stdc++.h>
using namespace std;

int n,m,a,b,ans[505][505];
const int ax[5] = {1,-1,0,0};
const int ay[5] = {0,0,1,-1};
struct peo
{
    int x,y;
};
queue <peo> bfs;
int main()
{
    cin >>n>>m>>a>>b;
    int ix,iy;
    for(int i = 0; i < a; i++)
    {
        cin >>ix>>iy;
        ans[ix][iy] = 0;
        bfs.push({ix,iy});
    }
    for(int i = 1; i <= n; i++)
    {
        for(int j = 1; j <= m; j++)
        {
            ans[i][j] = 1e9;
        }
    }
    while(!bfs.empty())
    {
        peo now = bfs.front();
        //cout <<now.x<<' '<<now.y<<"\n";
        bfs.pop();
        for(int i = 0; i < 4; i++)
        {
            int nx = now.x + ax[i],ny = now.y + ay[i];
            if(nx > n || nx < 1 || ny > m || ny < 1)continue;
            if(ans[nx][ny] == 1e9)
            {
                bfs.push({nx,ny});
                ans[nx][ny] = ans[now.x][now.y] + 1;
            }
        }
    }
    int ox,oy;
    for(int i = 0; i < b; i++)
    {
        cin >>ox>>oy;
        cout <<ans[ox][oy]-1e9<<"\n";
    }
}