#include <bits/stdc++.h>
using namespace std;
using LL = long long;

#define fs first
#define sc second

const LL MOD = 1e9+7;
const double EPS = 1e-10;

template<class T>bool chmax(T &a, const T &b) {
    if (a < b) { a = b; return true; }
    return false;
}
template<class T>bool chmin(T &a, const T &b) {
    if (b < a) { a = b; return true; }
    return false;
}

// std::vector Declaration
template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts) {
    return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template<typename T>
vector<T> make_vector(size_t a, T x) { return vector<T>(a, x); }
template<typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
    return vector<decltype(make_vector<T>(b,ts...))>(a,make_vector<T>(b,ts...));
}

// std::vector Input
template<typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (auto &e : v) {
        is >> e;
    }
    return is;
}

// std::vector Debug
template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::array Debug
template<typename T, size_t n>
ostream& operator<<(ostream& os, const array<T, n>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::deque Debug
template<typename T>
ostream& operator<<(ostream& os, const deque<T>& d) {
    os << "[";
    bool a = 1;
    for (auto e : d) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::pair Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
    os << "(" << p.first << " " << p.second << ")";
    return os;
}

// std::set Debug
template<typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::multiset Debug
template<typename T>
ostream& operator<<(ostream& os, const multiset<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::map Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const map<T, U>& mp) {
    os << "{";
    bool a = 1;
    for (auto e : mp) {
        os << (a ? "" : " ");
        os << e.first << ":" << e.second;
        a = 0;
    }
    os << "}";
    return os;
}

// std::tuple Debug
template<int N, class Tuple>
void out(ostream& os, const Tuple& t){}
template<int N, class Tuple, class H, class ...Ts>
void out(ostream& os, const Tuple& t) {
    if (N) os << " ";
    os << get<N>(t);
    out<N+1,Tuple,Ts...>(os, t);
}
template<class ...Ts>
ostream& operator<<(ostream& os, const tuple<Ts...>& t) {
    os << "(";
    out<0,tuple<Ts...>,Ts...>(os, t);
    os << ")";
    return os;
}

// Debug
#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl

// Weighted edge
template<typename T>
struct edge {
    int src, to;
    T cost;

    edge(int to, T cost) : src(-1), to(to), cost(cost) {}
    edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

    edge &operator=(const int &x) {
        to = x;
        return *this;
    }

    operator int() const { return to; }

    friend ostream& operator<<(ostream& os, const edge& e) {
        return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
    }
};

template<typename T>
using Edges = vector<edge<T>>;
template<typename T>
using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template<typename T>
using MatrixGraph = vector<vector<T>>;

void dfs(const auto& G, const auto& center,
         auto& sz, auto& g, int v) {
    int total_sz = 1;
    double total = center[v];
    for (auto u : G[v]) {
        dfs(G, center, sz, g, u);
        total_sz += sz[u];
        total += sz[u] * g[u];
    }
    sz[v] = total_sz;
    g[v] = total / total_sz;
}

int main()
{
    while (true) {
        int w, h; cin >> w >> h;
        if (w == 0 || h == 0) {
            break;
        }
        vector<string> p(h);
        for (int i = 0; i < h; ++i) {
            cin >> p[i];
        }

        auto piece = make_vector<int>(h, w, -1);
        int cnt = 0;
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                if (piece[i][j] != -1) {
                    continue;
                }
                if (p[i][j] == '.') {
                    continue;
                }
                queue<pair<int,int>> que;
                que.emplace(i, j);
                while (!que.empty()) {
                    int y, x; tie(y, x) = que.front(); que.pop();
                    piece[y][x] = cnt;
                    int dy[4] = {0, 1, 0, -1},
                        dx[4] = {-1, 0, 1, 0};
                    for (int k = 0; k < 4; ++k) {
                        int ty = y + dy[k],
                            tx = x + dx[k];
                        if (ty < 0 || h <= ty || tx < 0 || w <= tx) {
                            continue;
                        }
                        if (piece[ty][tx] != -1) continue;
                        if (p[i][j] != p[ty][tx]) continue;
                        que.emplace(ty, tx);
                    }
                }
                ++cnt;
            }
        }

        vector<int> par(cnt, -1);
        for (int i = 0; i < h-1; ++i) {
            for (int j = 0; j < w; ++j) {
                if (piece[i][j] == -1 ||
                    piece[i+1][j] == -1) continue;
                if (piece[i][j] != piece[i+1][j]) {
                    par[piece[i][j]] = piece[i+1][j];
                }
            }
        }

        UnWeightedGraph G(cnt);
        for (int i = 0; i < cnt-1; ++i) {
            G[par[i]].push_back(i);
        }

        int root = -1;
        for (int j = 0; j < w; ++j) {
            if (piece[h-1][j] != -1) {
                root = piece[h-1][j];
                break;
            }
        }

        vector<vector<pair<int,int>>> shape(cnt);
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                if (piece[i][j] == -1) continue;
                shape[piece[i][j]].emplace_back(i, j);
            }
        }
        vector<double> center(cnt);
        for (int i = 0; i < cnt; ++i) {
            double x = 0;
            for (auto p : shape[i]) {
                x += p.sc;
            }
            center[i] = x / 4;
        }

        vector<int> sz(cnt);
        vector<double> g(cnt);

        dfs(G, center, sz, g, root);

        bool ans = true;
        for (int i = 0; i < cnt; ++i) {
            int l = w-1, r = 0;
            for (auto p : shape[i]) {
                int y = p.fs, x = p.sc;
                if (i == cnt-1) {
                    if (y == h-1) {
                        chmin(l, x); chmax(r, x);
                    }
                } else {
                    if (piece[y+1][x] == par[i]) {
                        chmin(l, x); chmax(r, x);
                    }
                }
            }
            if (l-0.5 < g[i] && g[i] < r+0.5) {
                continue;
            }
            ans = false;
            break;
        }

        cout << (ans ? "STABLE" : "UNSTABLE") << endl;
    }

    return 0;
}



Measured memory_bytes: 3232