为什么50分

#include<bits/stdc++.h>
using namespace std;
int n,vis[1005][1005],mp[1005][1005];
int x1,y3,x2,y2;
struct node{
    int x,y,step;
};
queue<node>q;
int xx[4]={1,0,-1,0};
int yy[4]={0,1,0,1};
int main(){
    cin>>n;
    char b;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            cin>>b;
            if(b=='1')mp[i][j]=1;
            else  mp[i][j]=0;

        }
    }
    cin>>x1>>y3>>x2>>y2;
    q.push(node{x1,y3,0});
    vis[x1][y3]=true;   
    mp[x2][y2]=1e9;
    while(!q.empty()){
        for(int i=0;i<4;i++){
            int dx=xx[i]+q.front().x;
            int dy=yy[i]+q.front().y;   
            if(dx==x2&&dy==y2){
//                 cout<<dx<<" "<<dy<<" "<<q.front().step+1<<" "<<mp[x2][y2]<<endl;
                    mp[x2][y2]=min(mp[x2][y2],q.front().step+1);
            }
            if(dx>=1&&dx<=n&&dy>=1&&dy<=n&&!vis[dx][dy]&&mp[dx][dy]==0){
                q.push(node{dx,dy,q.front().step+1});
                vis[dx][dy]=1;
            }
        } 
        q.pop();
    }
    cout<<mp[x2][y2];
    return 0;
}