Grid [Gym-100819O] [Problem D]

#include <iostream>
#include <cstdio>
#include<queue>
#include<cstring>
#include <algorithm>
using namespace std;
int G[501][501];
bool vis[501][501];
int dx[4] = {-1,1,0,0};
int dy[4] = {0,0,1,-1};
typedef struct
{
    int x,y,time;
}node;
void bfs(int m,int n)
{
    node s;
    s.x = 0;
    s.y = 0;
    s.time = 0;
    queue<node> q;
    q.push(s);
    memset(vis,false,sizeof(vis));
    vis[0][0] = true;
    while(!q.empty())
    {
        node now = q.front(); q.pop();
        //printf("%d %d\n",now.x,now.y);
        if(now.x == m-1 && now.y==n-1) {
            printf("%d",now.time);
            return;
        }
        for(int i=0;i<4;i++)
        {
            int nx = now.x + dx[i]*G[now.x][now.y];
            int ny = now.y + dy[i]*G[now.x][now.y];
            //printf("-%d %d\n",nx,ny);
            if(nx<0||nx>=m||ny<0||ny>=n||vis[nx][ny]) continue;
            node nn;
            nn.x = nx; nn.y = ny; nn.time = now.time+1;
            q.push(nn);
            vis[nx][ny] = true;
        }
    }
    printf("IMPOSSIBLE");
}

int main()
{
    int m,n;cin>>m>>n;
    for(int i=0;i<m;i++)
    {
        string s;
        cin>>s;
        int len = s.length();
        for(int j=0;j<len;j++)
        {
            G[i][j] = s[j]-'0';
        }
    }
    //puts("here");
    bfs(m,n);
}