Language: C++

//I'll always miss you like a darling.
#include <bits/stdc++.h>
#include <unordered_set>
using namespace std;

#define LL long long
#define LD long double
#define ull unsigned long long
#define x first
#define y second
#define pb push_back
#define pf push_front
#define mp make_pair
#define Pair pair<int,int>
#define pLL pair<LL,LL>
#define pii pair<double,double>
#define LOWBIT(x) x & (-x)

#define rep(i,a,b) for (int i=a;i<=b;i++)
#define REP(i,a,b) for (int i=a;i>=b;i--)

const int INF=2e9;
const LL LINF=2e16;
const int magic=348;
const int MOD=1e9+7;
const double eps=1e-10;
const double pi=acos(-1);

struct fastio
{
    static const int S=1e7;
    char rbuf[S+48],wbuf[S+48];int rpos,wpos,len;
    fastio() {rpos=len=wpos=0;}
    inline char Getchar()
    {
        if (rpos==len) rpos=0,len=fread(rbuf,1,S,stdin);
        if (!len) return EOF;
        return rbuf[rpos++];
    }
    template <class T> inline void Get(T &x)
    {
        char ch;bool f;T res;
        while (!isdigit(ch=Getchar()) && ch!='-') {}
        if (ch=='-') f=false,res=0; else f=true,res=ch-'0';
        while (isdigit(ch=Getchar())) res=res*10+ch-'0';
        x=(f?res:-res);
    }
    inline void getstring(char *s)
    {
        char ch;
        while ((ch=Getchar())<=32) {}
        for (;ch>32;ch=Getchar()) *s++=ch;
        *s='\0';
    }
    inline void flush() {fwrite(wbuf,1,wpos,stdout);fflush(stdout);wpos=0;}
    inline void Writechar(char ch)
    {
        if (wpos==S) flush();
        wbuf[wpos++]=ch;
    }
    template <class T> inline void Print(T x,char ch)
    {
        char s[20];int pt=0;
        if (x==0) s[++pt]='0';
        else
        {
            if (x<0) Writechar('-'),x=-x;
            while (x) s[++pt]='0'+x%10,x/=10;
        }
        while (pt) Writechar(s[pt--]);
        Writechar(ch);
    }
    inline void printstring(char *s)
    {
        int pt=1;
        while (s[pt]!='\0') Writechar(s[pt++]);
    }
}io;

template<typename T> inline void check_max(T &x,T cmp) {x=max(x,cmp);}
template<typename T> inline void check_min(T &x,T cmp) {x=min(x,cmp);}
template<typename T> inline T myabs(T x) {return x>=0?x:-x;}
template<typename T> inline T gcd(T x,T y) {return y==0?x:gcd(y,x%y);}
inline int add(int x) {if (x>=MOD) x-=MOD;return x;}
inline int add(int x,int MO) {if (x>=MO) x-=MO;return x;}
inline int sub(int x) {if (x<0) x+=MOD;return x;}
inline int sub(int x,int MO) {if (x<0) x+=MO;return x;}
inline void Add(int &x,int y) {x=add(x+y);}
inline void Add(int &x,int y,int MO) {x=add(x+y,MO);}
inline void Sub(int &x,int y) {x=sub(x-y);}
inline void Sub(int &x,int y,int MO) {x=sub(x-y,MO);}
template<typename T> inline int quick_pow(int x,T y) {int res=1;while (y) {if (y&1) res=1ll*res*x%MOD;x=1ll*x*x%MOD;y>>=1;}return res;}
template<typename T> inline int quick_pow(int x,T y,int MO) {int res=1;while (y) {if (y&1) res=1ll*res*x%MO;x=1ll*x*x%MO;y>>=1;}return res;}

const int MAXN=1000;

int n,m,sx,sy,ex,ey;

char Map[MAXN+48][MAXN+48];

int dx[]={-1,0,1,0},dy[]={0,1,0,-1};

int index,id[MAXN+48][MAXN+48];
unordered_set<int> v[1000048];

inline void dfs(int x,int y,int Id)
{
    id[x][y]=Id;
    rep(dir,0,3)
    {
        int xx=x+dx[dir],yy=y+dy[dir];
        if (1<=xx && xx<=n && 1<=yy && yy<=m && Map[xx][yy]=='.' && !id[xx][yy]) dfs(xx,yy,Id);
    }
}

int dist[1000048];

int q[1000048],head,tail;

inline void bfs()
{
    rep(i,1,index) dist[i]=INF;
    dist[id[sx][sy]]=0;head=tail=1;q[1]=id[sx][sy];
    while (head<=tail)
    {
        int cur=q[head++];
        for (unordered_set<int>::iterator iter=v[cur].begin();iter!=v[cur].end();iter++)
        {
            int to=*iter;
            if (dist[to]>=INF)
            {
                dist[to]=dist[cur]+1;
                q[++tail]=to;
            }
        }
    }
}

int main ()
{
#ifndef ONLINE_JUDGE
    double TIME=clock();
    freopen ("a.in","r",stdin);
    freopen ("a.out","w",stdout);
    cerr<<"Running..."<<endl;
#endif
    scanf("%d%d",&n,&m);
    scanf("%d%d%d%d",&sx,&sy,&ex,&ey);
    rep(i,1,n) scanf("%s",Map[i]+1);
    index=0;rep(i,1,n) rep(j,1,m) if (Map[i][j]=='.' && !id[i][j]) ++index,dfs(i,j,index);
    rep(i,1,n) rep(j,1,m)
    {
        if (Map[i][j]!='.') continue;
        rep(d1,-2,2) rep(d2,-2,2)
        {
            int xx=i+d1,yy=j+d2;
            if (1<=xx && xx<=n && 1<=yy && yy<=m && Map[xx][yy]=='.' && id[i][j]!=id[xx][yy] && v[id[i][j]].find(id[xx][yy])==v[id[i][j]].end())
                v[id[i][j]].insert(id[xx][yy]),v[id[xx][yy]].insert(id[i][j]);
        }
    }
    bfs();
    if (dist[id[ex][ey]]>=INF) puts("-1"); else printf("%d\n",dist[id[ex][ey]]);
#ifndef ONLINE_JUDGE
    cerr<<"Exec Time: "<<(clock()-TIME)/CLOCKS_PER_SEC<<endl;
#endif
    return 0;
}

Compiler output:
a.cc:103:5: error: 'int index' redeclared as different kind of entity
  103 | int index,id[MAXN+48][MAXN+48];
      |     ^~~~~
In file included from /usr/include/string.h:462,
                 from /usr/include/c++/14/cstring:43,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:121,
                 from a.cc:2:
/usr/include/strings.h:50:20: note: previous declaration 'const char* index(const char*, int)'
   50 | extern const char *index (const char *__s, int __c)
      |                    ^~~~~
a.cc: In function 'void bfs()':
a.cc:19:34: error: invalid operands of types 'int' and '<unresolved overloaded function type>' to binary 'operator<='
   19 | #define rep(i,a,b) for (int i=a;i<=b;i++)
      |                                  ^
a.cc:122:5: note: in expansion of macro 'rep'
  122 |     rep(i,1,index) dist[i]=INF;
      |     ^~~
a.cc: In function 'int main()':
a.cc:150:11: error: overloaded function with no contextual type information
  150 |     index=0;rep(i,1,n) rep(j,1,m) if (Map[i][j]=='.' && !id[i][j]) ++index,dfs(i,j,index);
      |           ^
a.cc:150:70: error: no pre-increment operator for type
  150 |     index=0;rep(i,1,n) rep(j,1,m) if (Map[i][j]=='.' && !id[i][j]) ++index,dfs(i,j,index);
      |                                                                      ^~~~~
a.cc:150:79: error: cannot resolve overloaded function 'index' based on conversion to type 'int'
  150 |     index=0;rep(i,1,n) rep(j,1,m) if (Map[i][j]=='.' && !id[i][j]) ++index,dfs(i,j,index);
      |                                                                            ~~~^~~~~~~~~~~