#include <cassert>// c
#include <iostream>// io
#include <iomanip>
#include <fstream>
#include <sstream>
#include <vector>// container
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <algorithm>// other
#include <complex>
#include <numeric>
#include <functional>
#include <random>
#include <regex>
using namespace std;

typedef long long  ll;

#define ALL(c) (begin(c)),(end(c))
#define REP(i,n) FOR(i,0,n)
#define REPr(i,n) FORr(i,0,n)
#define FOR(i,l,r) for(int i=(int)(l);i<(int)(r);++i)
#define FORr(i,l,r) for(int i=(int)(r)-1;i>=(int)(l);--i)
#define EACH(it,o) for(auto it = (o).begin(); it != (o).end(); ++it)
#define IN(l,v,r) ((l)<=(v) && (v)<(r))
#define UNIQUE(v) v.erase(unique(ALL(v)),v.end())
//debug
#define DUMP(x)  cerr << #x << " = " << (x)
#define LINE()    cerr<< " (L" << __LINE__ << ")"

class range {
private:
	struct Iter{
		int v;
		int operator*(){return v;}
		bool operator!=(Iter& itr) {return v < itr.v;}
		void operator++() {++v;}
	};
	Iter i, n;
public:
	range(int n) : i({0}), n({n}) {}
	range(int i, int n) : i({i}), n({n}) {}
	Iter& begin() {return i;}
	Iter& end() {return n;}
};

//output
template<typename T> ostream& operator << (ostream& os, const vector<T>& as){REP(i,as.size()){if(i!=0)os<<" "; os<<as[i];}return os;}
template<typename T> ostream& operator << (ostream& os, const vector<vector<T>>& as){REP(i,as.size()){if(i!=0)os<<endl; os<<as[i];}return os;}
template<typename T> ostream& operator << (ostream& os, const set<T>& ss){for(auto a:ss){if(a!=ss.begin())os<<" "; os<<a;}return os;}
template<typename T1,typename T2> ostream& operator << (ostream& os, const pair<T1,T2>& p){os<<p.first<<" "<<p.second;return os;}
template<typename K,typename V> ostream& operator << (ostream& os, const map<K,V>& m){bool isF=true;for(auto& p:m){if(!isF)os<<endl;os<<p;isF=false;}return os;}
template<typename T1> ostream& operator << (ostream& os, const tuple<T1>& t){os << get<0>(t);return os;}
template<typename T1,typename T2> ostream& operator << (ostream& os, const tuple<T1,T2>& t){os << get<0>(t)<<" "<<get<1>(t);return os;}
template<typename T1,typename T2,typename T3> ostream& operator << (ostream& os, const tuple<T1,T2,T3>& t){os << get<0>(t)<<" "<<get<1>(t)<<" "<<get<2>(t);return os;}
template<typename T1,typename T2,typename T3,typename T4> ostream& operator << (ostream& os, const tuple<T1,T2,T3,T4>& t){os << get<0>(t)<<" "<<get<1>(t)<<" "<<get<2>(t)<<" "<<get<3>(t);return os;}
template<typename T1,typename T2,typename T3,typename T4,typename T5> ostream& operator << (ostream& os, const tuple<T1,T2,T3,T4,T5>& t){os << get<0>(t)<<" "<<get<1>(t)<<" "<<get<2>(t)<<" "<<get<3>(t)<<" "<<get<4>(t);return os;}
template<typename T1,typename T2,typename T3,typename T4,typename T5,typename T6> ostream& operator << (ostream& os, const tuple<T1,T2,T3,T4,T5,T6>& t){os << get<0>(t)<<" "<<get<1>(t)<<" "<<get<2>(t)<<" "<<get<3>(t)<<" "<<get<4>(t)<<" "<<get<5>(t);return os;}
template<typename T1,typename T2,typename T3,typename T4,typename T5,typename T6,typename T7> ostream& operator << (ostream& os, const tuple<T1,T2,T3,T4,T5,T6,T7>& t){os << get<0>(t)<<" "<<get<1>(t)<<" "<<get<2>(t)<<" "<<get<3>(t)<<" "<<get<4>(t)<<" "<<get<5>(t)<<" "<<get<6>(t);return os;}

//input
char tmp[1000];
#define nextInt(n) scanf("%d",&n)
#define nextLong(n) scanf("%lld",&n) //I64d
#define nextDouble(n) scanf("%lf",&n) 
#define nextChar(n) scanf("%c",&n)
#define nextString(n) scanf("%s",tmp);n=tmp

// values
template<typename T> T INF(){assert(false);};
template<> int INF<int>(){return 1<<28;};
template<> ll INF<ll>(){return 1LL<<58;};
template<> double INF<double>(){return 1e16;};

template<class T> T EPS(){assert(false);};
template<> int EPS<int>(){return 1;};
template<> ll EPS<ll>(){return 1LL;};
template<> double EPS<double>(){return 1e-8;};
template<> long double EPS<long double>(){return 1e-8;};

template<typename T,typename U> T pfmod(T v,U M){return fmod(fmod(v,M)+M,M);}

ll gcd_positive(ll a,ll b) { return b == 0 ? a : gcd_positive(b,a%b); }
ll gcd(ll a,ll b) { return gcd_positive(abs(a), abs(b)); }
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}

namespace EGraph{
	typedef ll Cost;Cost CINF=INF<Cost>();
	struct Edge{
		int from,to;Cost cost;
		Edge(int from,int to,Cost cost)
		: from(from),to(to),cost(cost) {};
		bool operator<(Edge r) const{ return cost<r.cost;}
		bool operator>(Edge r) const{ return cost>r.cost;}
	};
	typedef vector<vector<Edge> >  Graph;
}
using namespace EGraph;


int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};

class Main{

	typedef ll Cost;

	Cost minimum_steiner_tree(const vector<int>& ts, const vector<vector<Cost>> &g) {
		const int n = g.size();
		if (ts.size() <= 1) return 0;

		// all-pair shortest
		vector<vector<Cost>> ds=g;
		REP(k,n)REP(i,n)REP(j,n) ds[i][j] = min(ds[i][j], ds[i][k] + ds[k][j]);

		vector<vector<Cost>> dp(1<<ts.size(),vector<ll>(n,INF<Cost>()));
		

		REP(i,ts.size())REP(p,n){
			REP(q,n) dp[1<<i][p]=min(dp[1<<i][p],ds[p][q]+ds[ts[i]][q]);
		}
		REP(i,ts.size())REP(j,ts.size())REP(p,n)REP(q,n){
			dp[(1<<i | 1<< j)][p]=min(dp[(1<<i | 1<< j)][p],ds[p][q]+ds[ts[i]][q]+ds[ts[j]][q]);
		}

		// // trivial case
		REP(i,ts.size())REP(q,n) dp[1 << i][q] = ds[ts[i]][q];

		REP(S,1<<ts.size())if(S&(S-1)){
			REP(p,n)REP(E,S)if((E|S) == S) dp[S][p]=min(dp[S][p],dp[E][p] + dp[S-E][p]);

			REP(p,n)REP(q,n) dp[S][p] = min(dp[S][p], dp[S][q] + ds[p][q] );
		}

		Cost res =INF<Cost>();
		REP(q,n)res = min(res,dp[((1 << ts.size())-1)][q]);

		return res;
	}


	public:
	void run(){
		while(true){
			int H,W;cin >> H >>W;if(H==0 && W==0)break;
			vector<vector<int>> board(H,vector<int>(W));
			REP(y,H)REP(x,W)cin >> board[y][x];

			vector<vector<ll>> mat(H*W,vector<ll>(H*W,INF<ll>()));
			REP(i,H*W)mat[i][i]=0;

			vector<int> vs;
			REP(y,H)REP(x,W)if(board[y][x])vs.push_back(y*W+x);
			
			REP(y,H)REP(x,W){
				REP(d,4){
					int ny=y+dy[d],nx=x+dx[d];
					if(!IN(0,nx,W) || !IN(0,ny,H))continue;
					mat[y*W+x][ny*W+nx]=1;
				}
			}
			cout << H*W - (1+minimum_steiner_tree(vs,mat)) <<endl;

		}
	}
};

int main(){
	cout <<fixed<<setprecision(20);
	cin.tie(0);
	ios::sync_with_stdio(false);
	Main().run();
	return 0;
}

Measured memory_bytes: 1632