#include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>
#include <fstream>
#include <cassert>
using namespace std;

//common

typedef int  i32;
typedef long long i64,ll;
typedef long double ld;

#define BR "\n"
#define ALL(c) (c).begin(),(c).end()
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define FOR(i,l,r) for(int i=(int)l;i<(int)(r);++i)
#define EACH(it,o) for(auto it = (o).begin(); it != (o).end(); ++it)
#define IN(l,v,r) ((l)<=(v) && (v)<(r))

//config
//#define NDEBUG
//#define INF 1<<30
//#define EPS 1e-8
//const ll MOD =100000007;

//debug
#ifdef NDEBUG
#define DUMP(x)
#define DUMPLN(x)
#define DEBUG(x)
#define DEBUGLN(x)
#define LINE()
#define LINELN()
#define CHECK(exp,act)
#define STOP(e)
#else
#define DUMP(x)  cerr << #x << " = " << (x)
#define DUMPLN(x)  DUMP(x) <<endl
#define DEBUG(x) DUMP(x) << LINE() << " " << __FILE__
#define DEBUGLN(x) DEBUG(x)<<endl
#define LINE()    cerr<< " (L" << __LINE__ << ")"
#define LINELN()    LINE()<<endl
#define CHECK(exp,act)  if(exp!=act){DUMPLN(exp);DEBUGLN(act);}
#define STOP(e)  CHECK(e,true);if(!(e)) exit(1);
#endif

template<class T> inline string toString(const vector<T>& x) {
	stringstream ss;
	REP(i,x.size()){
		if(i!=0)ss<<" ";
		ss<< x[i];
	}
	return ss.str();
}

template<class T> inline string toString(const vector<vector<T> >& map) {
	stringstream ss;
	REP(i,map.size()){
		if(i!=0)ss<<BR;
		ss<< toString(map[i]);
	}
	return ss.str();
}
template<class K,class V>  string toString(map<K,V>& x) {
	string res;stringstream ss;
	for(auto& p:x)ss<< p.first<<":" << p.second<<" ";
	return ss.str();
}

string BITtoString(int bit){
    stringstream ss;
    while(bit!=0){ss<<(bit%2);bit/=2;}
    string res=ss.str();reverse(ALL(res));
    return res;
}

template<typename T,typename V> inline T pmod(T v,V MOD){
	return (v%MOD+MOD)%MOD;
}
#define nextInt(n) scanf("%d",&n)
#define defInt(n) int n;nextInt(n)
#define nextLong(n) scanf("%lld",&n)
#define defLong(n) ll n;nextLong(n)
#define nextDouble(n) scanf("%lf",&n)

struct UnionFind{
    vector<int> par; // 親
    vector<int> rank; // 木の深さ
    vector<int> ss;//xの属する集合のサイズ
    int size;//集合の個数
    UnionFind(int n){
        REP(i,n) par.push_back(i);
        rank = vector<int>(n);
        ss=vector<int>(n,1);
        size=n;
    }
    int root(int x){
        if(par[x] == x)return x;
        return par[x] = root(par[x]);
    }
    bool same(int x,int y){
        return root(x) == root(y);
    }
    void unite(int x,int y){
        x = root(x);y = root(y);
        if(x==y)return;
        if(rank[x] < rank[y]){
            par[x] = y;
            ss[y]+=ss[x];
        }else{
            par[y] = x;
            ss[x]+=ss[y];
        }
        if(rank[x] == rank[y]) rank[x]++;
        size--;
    }
    int getS(int x){
        return ss[root(x)];
    }
};

int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
int INF=1<<28;
class Main{
	public:

	int w,h;

	int enc(int y,int x){
		return y*w+x;
	}

	void run(){
		// ifstream cin("D2");
//		ofstream cout( "D2.out" );
		
		while(true){
			cin >> w  >> h;if(w==0 && h==0)break;
			vector<string> board(h);REP(y,h) cin >> board[y];

			UnionFind uf(w*h);//ブロックグループ
			REP(y,h)REP(x,w){
				REP(d,4){
					int ny=y+dy[d],nx=x+dx[d];
					if(!IN(0,ny,h) || !IN(0,nx,w))continue;
					if(board[ny][nx]==board[y][x])
						uf.unite(enc(y,x),enc(ny,nx));
				}
			}

			//木の生成
			chs=vector<vector<int>>(w*h);
			lx=vector<int>(w*h,INF);rx=vector<int>(w*h,-INF);
			bs=vector<vector<int>>(w*h,vector<int>(w));
		
			//ルート
			REP(x,w)if(board[h-1][x]!='.'){
				root=uf.root(enc(h-1,x));
				lx[uf.root(enc(h-1,x))]=min(lx[uf.root(enc(h-1,x))],x);rx[uf.root(enc(h-1,x))]=max(rx[uf.root(enc(h-1,x))],x+1);
			}
			//子
			REP(y,h)REP(x,w){
				int cy=y+dy[3],cx=x+dx[3];
				if(!IN(0,cy,h) || !IN(0,cx,w))continue;
				if(board[y][x]!=board[cy][cx] && board[y][x]!='.' && board[cy][cx]!='.'){
					chs[uf.root(enc(y,x))].push_back(uf.root(enc(cy,cx)));
					lx[uf.root(enc(cy,cx))]=min(lx[uf.root(enc(cy,cx))],x);rx[uf.root(enc(cy,cx))]=max(rx[uf.root(enc(cy,cx))],x+1);
				}
			}
			REP(y,h)REP(x,w){
				sort(ALL(chs[uf.root(enc(y,x))]));
				chs[uf.root(enc(y,x))].erase(unique(ALL(chs[uf.root(enc(y,x))])),chs[uf.root(enc(y,x))].end());
			}
	
			REP(y,h)REP(x,w)if(board[y][x]!='.')bs[uf.root(enc(y,x))][x]++;

			cout <<(dfs(root)?"STABLE":"UNSTABLE")<<endl;
		}
	}
	int root;		
	vector<int> lx,rx; 
	vector<vector<int>> bs; 
	vector<vector<int>> chs;

	bool dfs(int p){
		//check chd
		REP(i,chs[p].size()){
			if(!dfs(chs[p][i]))return false;
//			lx[p]=min(lx[p],lx[chs[p][i]]);rx[p]=max(rx[p],rx[chs[p][i]]);
			REP(x,w)bs[p][x]+=bs[chs[p][i]][x];
		}
//	DUMP(p);DUMP(lx[p]);DUMP(rx[p]);DUMP(BR);

		//check ok
		double m=0;int sum=0;
		REP(x,w){
			m+=(x+0.5)*bs[p][x];sum+=bs[p][x];
		}
		m/=sum;
//		DUMP(m);DUMP(BR);
		if(lx[p]<m && m<rx[p])return true;
		return false;
	}

};
int main(){
	cout <<fixed<<setprecision(15);	
	ios::sync_with_stdio(false);
	Main().run();
	return 0;
}

Measured memory_bytes: 1328