#include <bits/stdc++.h>
using namespace std;
// #define int long long
#define stoi stoll
using ll=long long;
using vi=vector<int>;
using pii=pair<int,int>;
#define ALL(c) begin(c),end(c)
#define RALL(c) rbegin(c),rend(c)
#define ITR(i,b,e) for(auto i=(b);i!=(e);++i)
#define FORE(x,c) for(auto &&x:c)
#define REPF(i,a,n) for(int i=a,i##len=(int)(n);i<i##len;++i)
#define REP(i,n) REPF(i,0,n)
#define REPR(i,n) for(int i=(int)(n);i>=0;--i)
#define SZ(c) ((int)c.size())
#define CONTAIN(c,x) (c.find(x)!=end(c))
#define INSEG(l,x,r) ((l)<=(x)&&(x)<(r))
#define dump(...)
#define pb push_back
#define _ 0
const signed INF_=1001001001; const long long INF=1001001001001001001LL;
const int DX[9]={0,1,0,-1,1,1,-1,-1,0},DY[9]={-1,0,1,0,-1,1,1,-1,0};
template<class T> ostream& operator<<(ostream &os,const vector<T> &v) {
    ITR(i,begin(v),end(v))os<<*i<<(i==end(v)-1?"":" ");return os;}
template<class T> istream& operator>>(istream &is,vector<T> &v) {
    ITR(i,begin(v),end(v)) is>>*i;return is;}
template<class T,class U> istream& operator>>(istream &is, pair<T,U> &p) {
    is>>p.first>>p.second;return is;}
template<class T, class U> bool chmax(T &a,const U &b){return a<b?a=b,1:0;}
template<class T, class U> bool chmin(T &a,const U &b){return a>b?a=b,1:0;}
template <class T> void PSUM(T& c) {partial_sum(begin(c), end(c), begin(c));}
template<class T> using heap=priority_queue<T,vector<T>,greater<T>>;
struct before_main_function {
    before_main_function() {
        cin.tie(0); ios::sync_with_stdio(false);
        cout << setprecision(15) << fixed;
        #define endl "\n"
    }
} before_main_function;
//------------------8<------------------------------------8<--------------------

signed main() {
    while (1) {
        int W, H;
        cin >> W >> H;
        if (W == 0 && H == 0) break;

        vector<string> P(H);
        cin >> P;
        vector<vector<int>> piece_id(H, vector<int>(W, -1));

        int idx = 1;
        {
            auto Q = P;
            REPR(i, H - 1) REP(j, W) {
                if (Q[i][j] == '.') continue;
                auto dfs = [&](auto f, int y, int x, int v) -> void {
                    Q[y][x] = '.';
                    piece_id[y][x] = idx;
                    REP(dir, 4) {
                        int yy = y + DY[dir];
                        int xx = x + DX[dir];
                        if (!INSEG(0, yy, H) || !INSEG(0, xx, W) || Q[yy][xx] != v) continue;
                        f(f, yy, xx, v);
                    }
                };
                dfs(dfs, i, j, Q[i][j]);
                idx++;
            }
        }

        vector<pii> lr(idx, {INF, -INF});
        vector<set<int>> g(idx);
        vector<valarray<int>> weight(idx, valarray<int>(0, W));

        REPR(i, H - 1) REP(j, W) {
            if (P[i][j] == '.') continue;
            int pid = piece_id[i][j];
            auto dfs = [&](auto f, int y, int x) -> void {
                P[y][x] = '.';
                weight[pid][x]++;
                
                if (y == H - 1 || (piece_id[y + 1][x] != pid && piece_id[y + 1][x] != -1)) {
                    chmin(lr[pid].first, x);
                    chmax(lr[pid].second, x);
                    if (y == H - 1) {
                        g[0].insert(pid);
                    } else {
                        g[piece_id[y + 1][x]].insert(pid);
                    }
                }

                REP(dir, 4) {
                    int yy = y + DY[dir];
                    int xx = x + DX[dir];
                    if (!INSEG(0, yy, H) || !INSEG(0, xx, W) || (P[yy][xx] == '.' || piece_id[yy][xx] != pid)) continue;
                    f(f, yy, xx);
                }
            };
            dfs(dfs, i, j);
        }

        bool stable = true;
        auto dfs = [&](auto f, int v) -> valarray<int> {
            auto &wei = weight[v];
            FORE(w, g[v]) {
                wei += f(f, w);
            }

            double g = 0;
            REP(i, W) g += i * wei[i];
            g += wei.sum() / 2.0;
            if (v > 0 && (g <= lr[v].first * wei.sum() || (lr[v].second + 1) * wei.sum() <= g)) {
                stable = false;
            }
            return wei;
        };
        dfs(dfs, 0);
        cout << (stable ? "STABLE" : "UNSTABLE") << endl;
    }
    return (0^_^0);
}




Measured memory_bytes: 3248