#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_DEPRECATE

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <limits>
#include <cfloat>
#include <ctime>
#include <cassert>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <numeric>
#include <list>
#include <iomanip>
#include <fstream>
#include <iterator>
#include <bitset>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> Pii;
typedef pair<ll, ll> Pll;

#define FOR(i,n) for(int i = 0; i < (n); i++)
#define sz(c) ((int)(c).size())
#define ten(x) ((int)1e##x)
#define tenll(x) ((ll)1e##x)

// #pragma comment(linker,"/STACK:36777216")

template<class T> void chmax(T& l, const T r) { l = max(l, r); }
template<class T> void chmin(T& l, const T r) { l = min(l, r); }
template<class T> T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
template<class T> T extgcd(T a, T b, T& x, T& y) { for (T u = y = 1, v = x = 0; a;) { T q = b / a; swap(x -= q * u, u); swap(y -= q * v, v); swap(b -= q * a, a); } return b; }
template<class T> T mod_inv(T a, T m) { T x, y; extgcd(a, m, x, y); return (m + x % m) % m; }
ll mod_pow(ll a, ll n, ll mod) { ll ret = 1; ll p = a % mod; while (n) { if (n & 1) ret = ret * p % mod; p = p * p % mod; n >>= 1; } return ret; }

#ifdef _WIN32
#   define mygc(c) (c)=getchar()
#   define mypc(c) putchar(c)
#else
#   define mygc(c) (c)=getchar_unlocked()
#   define mypc(c) putchar_unlocked(c)
#endif

void reader(int& x) { int k, m = 0; x = 0; for (;;) { mygc(k); if (k == '-') { m = 1; break; }if ('0' <= k&&k <= '9') { x = k - '0'; break; } }for (;;) { mygc(k); if (k<'0' || k>'9')break; x = x * 10 + k - '0'; }if (m) x = -x; }
void reader(ll& x) { int k, m = 0; x = 0; for (;;) { mygc(k); if (k == '-') { m = 1; break; }if ('0' <= k&&k <= '9') { x = k - '0'; break; } }for (;;) { mygc(k); if (k<'0' || k>'9')break; x = x * 10 + k - '0'; }if (m) x = -x; }
int reader(char c[]) { int i, s = 0; for (;;) { mygc(i); if (i != ' '&&i != '\n'&&i != '\r'&&i != '\t'&&i != EOF) break; }c[s++] = i; for (;;) { mygc(i); if (i == ' ' || i == '\n' || i == '\r' || i == '\t' || i == EOF) break; c[s++] = i; }c[s] = '\0'; return s; }
template <class T, class S> void reader(T& x, S& y) { reader(x); reader(y); }
template <class T, class S, class U> void reader(T& x, S& y, U& z) { reader(x); reader(y); reader(z); }
template <class T, class S, class U, class V> void reader(T& x, S& y, U& z, V & w) { reader(x); reader(y); reader(z); reader(w); }

void writer(int x, char c) { int s = 0, m = 0; char f[10]; if (x<0)m = 1, x = -x; while (x)f[s++] = x % 10, x /= 10; if (!s)f[s++] = 0; if (m)mypc('-'); while (s--)mypc(f[s] + '0'); mypc(c); }
void writer(ll x, char c) { int s = 0, m = 0; char f[20]; if (x<0)m = 1, x = -x; while (x)f[s++] = x % 10, x /= 10; if (!s)f[s++] = 0; if (m)mypc('-'); while (s--)mypc(f[s] + '0'); mypc(c); }
void writer(const char c[]) { int i; for (i = 0; c[i] != '\0'; i++)mypc(c[i]); }
void writer(const char x[], char c) { int i; for (i = 0; x[i] != '\0'; i++)mypc(x[i]); mypc(c); }
template<class T> void writerLn(T x) { writer(x, '\n'); }
template<class T, class S> void writerLn(T x, S y) { writer(x, ' '); writer(y, '\n'); }
template<class T, class S, class U> void writerLn(T x, S y, U z) { writer(x, ' '); writer(y, ' '); writer(z, '\n'); }
template<class T> void writerArr(T x[], int n) { if (!n) { mypc('\n'); return; }FOR(i, n - 1)writer(x[i], ' '); writer(x[n - 1], '\n'); }

const int dxy[] = { 0, 1, 0, -1, 0 };

char c[61][11];
int w, h;

bool ok;

Pii dfs(int ch, int cw){
	vector<Pii> blocks;
	char color = c[ch][cw];
	queue<Pii> q;
	q.push(Pii(ch, cw));
	while (!q.empty()) {
		Pii p = q.front(); q.pop();
		FOR(k, 4){
			int nh = p.first + dxy[k], nw = p.second + dxy[k + 1];
			if (nh < 0 || nh >= h || nw < 0 || nw >= w) continue;
			if (c[nh][nw] == color && find(blocks.begin(),blocks.end(),Pii(nh,nw)) == blocks.end()) {
				blocks.push_back(Pii(nh,nw));
				q.push(Pii(nh, nw));
			}
		}
	}

	int lx = w, rx = 0;
	Pii upper;
	for (auto& b : blocks) {
		if (b.first == h - 1 || c[b.first + 1][b.second] != color && c[b.first + 1][b.second] != '.') {
			lx = min(lx, b.second);
			rx = max(rx, b.second);
		}
		if (b.first != 0 && c[b.first - 1][b.second] != color && c[b.first - 1][b.second] != '.') {
			auto tmp = dfs(b.first - 1, b.second);
			upper.first += tmp.first;
			upper.second += tmp.second;
		}
	}
	rx++;

	Pii ret = upper;
	for (auto& b : blocks) {
		c[b.first][b.second] = '.';
		ret.first += b.second * 2 + 1;
		ret.second += 2;
	}

	if (lx * ret.second < ret.first && ret.first < rx * ret.second) {
		// ok |= true;
	} else {
		ok = false;
	}

	return ret;
}

void solve(){
	ok = true;
	FOR(i, w) {
		if (c[h - 1][i] == '.') continue;
		dfs(h - 1, i);
	}
	puts(ok ? "STABLE" : "UNSTABLE");
}

int main(){
	while (cin >> w >> h,w) {
		FOR(i, h) cin >> c[i];
		solve();
	}
}

Measured memory_bytes: 1184