救！为什么我这个RE了呀

#include <iostream>
#include <queue>
#include <cstring>
#include <algorithm>
#include <cstdio>  
using namespace std;
const int N = 1010;
typedef pair<int, int> PII;
queue <PII> q;
int p[N][N];
int x1, y1, x2,y2;
int dist[N][N];
int dx[] = { -1,0,1,0 };
int dy[] = { 0,-1,0,1 };
int n;
int BFS(int x1, int y1)
{
    memset(dist, -1, sizeof(dist));
    q.push({x1,y1});
    dist[x1][y1] = 0;
    while (q.size())
    {
        PII t = q.front();
        q.pop();
        for (int i = 1; i <= 4; i++)
        {
            int a = dx[i] + t.first; int b = t.second + dy[i];
            if (a<1 || a>n || b<1 || b>n) { continue; }
            if (p[a][b] == 1) { continue; }
            if (dist[a][b] != -1) { continue; }
            q.push({ a,b });
            dist[a][b] = dist[t.first][t.second] + 1;
            if (a == x2 && b == y2) { return dist[x2][y2]; }
        }
    }return dist[x2][y2];

}

int main()
{

     scanf("%d", &n);

    for (int i = 1; i <= n; i++)
    {
        for (int j = 1; j <= n; j++)
        {
            scanf("%1d", &p[i][j]);
        }
    }

    scanf("%d %d %d %d", &x1, &y1, &x2, &y2);
    printf("%d", BFS(x1, y1));
    return 0;
}