程序代码：

#include <iostream>
#include <memory.h>
#include <cstdio>
#include <queue>
using namespace std;
int w,h;
int x1,y1,x2,y2;
char str[80][80];
int vis[80][80],cnt;
int x[4] = {1, -1, 0, 0};
int y[4] = {0, 0, 1, -1};
queue<struct Node> q;
struct Node
{
    int x;
    int y;
};
struct Node tmp,top;
int main()
{
    while(cin>>w>>h,w&&h)
    {
        cnt=1;
        getchar();
        for(int i = 1; i <= h; i++)
            gets(&str[i][1]);
        for(int j=0; j<=w; j++)
            str[0][j]=str[h+1][j]=' ';
        for(int i=0; i<=h; i++)
            str[i][0]=str[i][w+1]=' ';
        while(cin>>y1>>x1>>y2>>x2)
        {
            if(x1==0&&x2==0&&y1==0&&y2==0)
                break;
            for(int i=0; i<=h+1; i++)
            {
                memset(str[i],0,sizeof(str[i]));
                memset(vis[i],-1,sizeof(vis[i]));
            }
            tmp.x=x1;
            tmp.y=y1;
            q.push(tmp);
            vis[x1][y1]=0;
            while(!q.empty())
            {
                top=q.front();
                q.pop();
                for(int i=0; i<4; i++)
                {
                    tmp.x=top.x+x[i];
                    tmp.y=top.y+y[i];
                    while(str[tmp.x][tmp.y]==' '&&top.x+x[i]>=0&&top.x+x[i]<=h+1&&top.y+y[i]>=0&&top.y+y[i]<=w+1&&!vis[top.x+x[i]][top.y+y[i]])
                    {
                        q.push(tmp);
                        vis[tmp.x][tmp.y]=vis[top.x][top.y]+1;
                        tmp.x+=x[i];
                        tmp.y+=y[i];
                    }
                }
            }
            if(vis[x2][y2]==-1)
                printf("Pair %d: impossible.\n", cnt++);
            else
                printf("Pair %d: %d segments.\n", cnt++, vis[x2][y2]);
        }
    }
    return 0;
}