////////////////////////////////////////
///  tu3 pro-con template            ///
////////////////////////////////////////
#include <cassert>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <functional>
#include <vector>
#include <queue>
#include <string>
#include <complex>
#include <stack>
#include <set>
#include <map>
#include <list>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <regex>
using namespace std;

//// MACRO ////
#define countof(a) (sizeof(a)/sizeof(a[0]))

#define REP(i,n) for (int i = 0; i < (n); i++)
#define RREP(i,n) for (int i = (n)-1; i >= 0; i--)
#define FOR(i,s,n) for (int i = (s); i < (n); i++)
#define RFOR(i,s,n) for (int i = (n)-1; i >= (s); i--)
#define pos(c,i) c.being() + (i)
#define allof(c) c.begin(), c.end()
#define aallof(a) a, countof(a)
#define partof(c,i,n) c.begin() + (i), c.begin() + (i) + (n)
#define apartof(a,i,n) a + (i), a + (i) + (n)
#define long long long

#define EPS 1e-9
#define INF (1L << 30)
#define LINF (1LL << 60)

#define PREDICATE(t,a,exp) [&](const t & a) -> bool { return exp; }
#define COMPARISON_T(t) bool(*)(const t &, const t &)
#define COMPARISON(t,a,b,exp) [&](const t & a, const t & b) -> bool { return exp; }
#define CONVERTER(TSrc,t,TDest,exp) [&](const TSrc &t)->TDest { return exp; }

inline int sign(double x) { return (abs(x) < EPS ? 0 : x > 0 ? 1 : -1); }
inline bool inRange(int val, int min, int max) { return val >= min && val < max; }
inline bool inRange(double val, double min, double max) { return val - min > -EPS && val - max < EPS; }
template <class T> struct vevector : public vector<vector<T>> { vevector(int n = 0, int m = 0, const T &initial = T()) : vector<vector<T>>(n, vector<T>(m, initial)) { } };
template <class T> struct vevevector : public vector<vevector<T>> { vevevector(int n = 0, int m = 0, int l = 0, const T &initial = T()) : vector<vevector<T>>(n, vevector<T>(m, l, initial)) { } };
template <class T> struct vevevevector : public vector<vevevector<T>> { vevevevector(int n = 0, int m = 0, int l = 0, int k = 0, const T &initial = T()) : vector<vevevector<T>>(n, vevevector<T>(m, l, k, initial)) { } };

//// i/o helper ////

#ifdef ccout
#  define cout ccout
#  define endl cendl
#endif

struct _Reader { template <class T> _Reader operator ,(T &rhs) { cin >> rhs; return *this; } };
struct _Writer { bool f; _Writer() : f(false) { } template <class T> _Writer operator ,(const T &rhs) { cout << (f ? " " : "") << rhs; f = true; return *this; } };
#define READ(t,...) t __VA_ARGS__; _Reader(), __VA_ARGS__
#define WRITE(...) _Writer(), __VA_ARGS__; cout << endl
#ifdef _DEBUG
#define DEBUG WRITE
inline void readfrom(string filename) { freopen(filename.c_str(), "r", stdin); }
inline void writeto(string filename) { freopen(filename.c_str(), "w", stdout); }
#else
#define DEBUG(...)
inline void readfrom(...) { }
inline void writeto(...) { }
#endif

template <class T> T read() { T t; cin >> t; return t; }
template <class T> vector<T> read(int n) { vector<T> v; REP(i, n) { v.push_back(read<T>()); } return v; }
template <class T> vevector<T> read(int n, int m) { vevector<T> v; REP(i, n) v.push_back(read<T>(m)); return v; }
template <class T> vevector<T> readjag() { return read<T>(read<int>()); }
template <class T> vevector<T> readjag(int n) { vevector<T> v; REP(i, n) v.push_back(readjag<T>()); return v; }

template <class T1, class T2> inline istream & operator >> (istream & in, pair<T1, T2> &p) { in >> p.first >> p.second; return in; }
template <class T1, class T2> inline ostream & operator << (ostream &out, pair<T1, T2> &p) { out << p.first << p.second; return out; }
template <class T> inline ostream & operator << (ostream &out, const vector<T> &v)
{
	ostringstream ss;
	for (auto x : v) ss << x << ' ';
	auto s = ss.str();
	out << s.substr(0, s.length() - 1) << endl;
	return out;
}

//// start up ////
void solve();


int main()
{
	solve();
	return 0;
}

////////////////////
/// template end ///
////////////////////

typedef pair<int, int> P;

int seedfill(vector<string> &field, vector<P> *painted, char self, char replace, int r, int c)
{
	int ret = 0;
	if (field[r][c] == self)
	{
		if (painted) painted->emplace_back(r, c);
		field[r][c] = replace; ret++;
		ret += seedfill(field, painted, self, replace, r + 1, c);
		ret += seedfill(field, painted, self, replace, r - 1, c);
		ret += seedfill(field, painted, self, replace, r, c + 1);
		ret += seedfill(field, painted, self, replace, r, c - 1);
	}
	return ret;
}

bool test(vector<string> &field, int r, int c, const vector<P> &parent, double &cog, int &children)
{
	vector<P> painted;
	char target = field[r][c];
	seedfill(field, &painted, field[r][c], '.', r, c);
	for (auto p : painted) if (field[p.first-1][p.second] != '.')
	{
		double childCog = 0;
		int childCount = 0;
		if (!test(field, p.first - 1, p.second, painted, childCog, childCount)) return false;
		cog += childCog * childCount;
		children += childCount;
	}
	DEBUG("target:", target);
	DEBUG("child cog:", cog, "count:", children);
	double cogself = accumulate(allof(painted), 0.0, [](double x, P p) { return x + p.second+0.5; }) / painted.size();
	DEBUG("self cog:", cogself, "count:", 1);
	cog += cogself; children++;
	
	cog = cog / children;
	DEBUG("total cog:", cog, "count:", children);

	int ll2 = INF;
	int rr2 = -INF;
	for (auto p : painted) if (count(allof(parent), make_pair(p.first + 1, p.second)))
	{
		ll2 = min(ll2, p.second);
		rr2 = max(rr2, p.second + 1);
	}
	DEBUG("llrr:", ll2, rr2);

	return cog > ll2 + EPS && cog < rr2 - EPS;
}

void solve()
{
	int testcases = INF;
	REP(testcase, testcases)
	{
		READ(int, w, h);
		if (!(w | h)) break;
		w += 2;
		vector<string> field;
		field.push_back(string(w, '.'));
		REP(_, h) field.push_back("." + read<string>() + ".");
		field.push_back(string(w, '.'));
		h += 2;

		bool result = false;
		vector<P> floor;
		REP(c, w) floor.emplace_back(h - 1, c);
		REP(c, w) if (field[h-2][c] != '.')
		{
			double cog = 0; int cc = 0;
			result = test(field, h - 2, c, floor, cog, cc);
		}
		WRITE(result ? "STABLE" : "UNSTABLE");
	}
}

Measured memory_bytes: 1236