#pragma GCC optimize("Ofast","unroll-loops")
#pragma GCC push_options
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<bits/stdc++.h>
#include <xmmintrin.h>
#include <immintrin.h>
using namespace::std;
__attribute__((constructor))void init(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
template<typename T>using pbds=__gnu_pbds::tree<T,__gnu_pbds::null_type,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T>using pbds_map=__gnu_pbds::tree<T,T,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T,typename E>using hash_map=__gnu_pbds::gp_hash_table<T,E>;
template<typename T>using pqueue =__gnu_pbds::priority_queue<T, less<T>,__gnu_pbds::rc_binomial_heap_tag>;
typedef long long lint;
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define MOD 1000000007
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
template<typename T>inline void numout(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i<INF/2?i:"INF";f=1;}cout<<endl;}
template<typename T>inline void numout2(T t){for(auto i:t)numout(i);}
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void _output(T t){bool f=0;for(int i=0;i<t.size();i++){cout<<f?"":" "<<t[i];f=1;}cout<<endl;}
template<typename T>inline void _output2(T t){for(int i=0;i<t.size();i++)output(t[i]);}
#define rep(i,n) for(lint i=0;i<lint(n);++i)
#define repi(i,a,b) for(int i=int(a);i<(int)(b);++i)
#define rrep(i,n) for(int i=int(n)-1;i>=0;--i)
#define rrepi(i,a,b) for(int i=int(b)-1;i>=int(a);--i)
#define irep(i) for(int i=0;;++i)
#define all(n) begin(n),end(n)
inline lint gcd(lint A,lint B){return B?gcd(B,A%B):A;}
inline lint lcm(lint A,lint B){return A/gcd(A,B)*B;}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
const vector<lint> dx={-1,1,0,0,1,1,-1,-1};
const vector<lint> dy={0,0,-1,1,1,-1,1,-1};
#define SUM(v) accumulate(all(v),0LL)
auto call=[](auto f,auto... args){return f(f,args...);};

class mint {
  using u64 = std::uint_fast64_t;
    public:
    u64 a;
    constexpr mint(const u64 x = 0)noexcept:a(x % MOD){}
    constexpr u64 &value()noexcept{return a;}
    constexpr const u64 &value() const noexcept {return a;}
    constexpr mint operator+(const mint rhs)const noexcept{return mint(*this) += rhs;}
    constexpr mint operator-(const mint rhs)const noexcept{return mint(*this)-=rhs;}
    constexpr mint operator*(const mint rhs) const noexcept {return mint(*this) *= rhs;}
    constexpr mint operator/(const mint rhs) const noexcept {return mint(*this) /= rhs;}
    constexpr mint &operator+=(const mint rhs) noexcept {
        a += rhs.a;
        if (a >= MOD)a -= MOD;
        return *this;
    }
    constexpr mint &operator-=(const mint rhs) noexcept {
        if (a<rhs.a)a += MOD;
        a -= rhs.a;
        return *this;
    }
    constexpr mint &operator*=(const mint rhs) noexcept {
        a = a * rhs.a % MOD;
        return *this;
    }
    constexpr mint operator++(int n) const noexcept {
        return mint(*this) +=1;
    }
    constexpr mint operator--(int n) const noexcept {
        return mint(*this) -=1;
    }
    constexpr mint &operator/=(mint rhs) noexcept {
        u64 exp=MOD-2;
        while (exp) {
            if (exp % 2) {
                *this *= rhs;
            }
            rhs *= rhs;
            exp /= 2;
        }
        return *this;
    }
    mint pow(long long n){
        long long x=a;
        mint ret = 1;
        while(n>0) {
            if(n&1)(ret*=x);
            (x*=x)%=MOD;
            n>>=1;
        }
        return ret;
    }
    mint inv()const{
        int _a=a,b=MOD,u=1,v=0,t;
        while(b>0) {
            t=_a/b;
            swap(_a-=t*b,b);
            swap(u-=t*v,v);
        }
        return mint(u);
    }
    lint comb(auto b){
        using lint=long long;
        static bool init=1;
        static lint fac[1000001],ifac[1000001];
        if(init){
            init=0;
            fac[0]=1;
            ifac[0]=1;
            auto mod_pow=[&](lint x,lint n){
                lint ans = 1;
                while(n != 0){
                    if(n&1)ans=ans*x%MOD;
                    x=x*x%MOD;
                    n=n>>1;
                }
                return ans;
            };
            for(int i=0;i<1000000;i++){
                fac[i+1]=fac[i]*(i+1)%MOD;
                ifac[i+1]=ifac[i]*mod_pow(i+1, MOD-2)%MOD;
            }
        }
        if(a==0&&b==0)return 1;
        if(a<b||a<0)return 0;
        lint tmp=ifac[a-b]*ifac[b]%MOD;
        return tmp*fac[a]%MOD;
    }
    
    friend ostream& operator<<(ostream& lhs, const mint& rhs) noexcept {
        lhs << rhs.a;
        return lhs;
    }
    friend istream& operator>>(istream& lhs,mint& rhs) noexcept {
        lhs >> rhs.a;
        return lhs;
    }
};
inline mint fact(lint a){return a?fact(a-1)*a:1;}

int main(){
    lint h,w;
    cin>>h>>w;
    vector<string>v(h);
    rep(i,h)cin>>v[i];
    lint ans=0;
    rep(i,h)rep(j,w){
        if(v[i][j]=='#')continue;
        queue<pair<lint,lint>>que;
        vector<lint>point(h*w,-1);
        que.emplace(i,j);
        point[i*w+j]=0;
        while(!que.empty()){
            lint x,y;
            cin>>x>>y;
            tie(x,y)=que.front();
            que.pop();
            rep(k,4){
                if(x+dx[k]<0||h<=x+dx[k]||y+dy[k]<0||w<=y+dy[k])continue;
                if(point[(x+dx[k])*w+y+dy[k]]!=-1)continue;
                if(v[(x+dx[k])][y+dy[k]]=='#')continue;
                point[(x+dx[k])*w+y+dy[k]]=point[x*w+y]+1;
                chmax(ans,point[x*w+y]+1);
                que.emplace(x+dx[k],y+dy[k]);
            }
        }
    }
    cout<<ans<<endl;
}

Measured memory_bytes: 256