using System;
using System.Linq;
using System.Collections.Generic;
using Debug = System.Diagnostics.Debug;
using SB = System.Text.StringBuilder;
using Number = System.Int64;
namespace Program
{
    public class Solver
    {
        public void Solve()
        {

            for (;;)
            {
                var m = ri;
                var n = ri + 1;
                if (m == 0) return;
                var t = Enumerate(n, x => x != 0 ? rs : new string('.', m));
                var s = new DisjointSet(n * m);
                for (int i = 0; i < n; i++)
                    for (int j = 0; j < m; j++)
                    {
                        for (int k = 0; k < 4; k++)
                        {
                            var ni = i + dx[k];
                            var nj = j + dy[k];
                            if (ni < 0 || ni >= n || nj < 0 || nj >= m) continue;
                            if (char.IsDigit(t[i][j]) && t[i][j] == t[ni][nj])
                                s.Unite(i * m + j, ni * m + nj);
                        }
                    }
                var ok = true;
                Func<int, KeyValuePair<int, int>> dfs = null;
                var G = Enumerate(n * m, x => new List<int>());
                for (int i = 0; i < n * m; i++)
                    if (s.Size(i) > 1) G[s[i]].Add(i);
                var vis = new bool[n * m];
                dfs = cur =>
                  {
                      if (vis[s[cur]]) return new KeyValuePair<int, int>(0, 0);
                      vis[s[cur]] = true;
                      var xsum = 0;
                      var sum = 0;
                      var xmin = 1000000;
                      var xmax = 0;
                      foreach (var x in G[s[cur]])
                      {
                          if (x + m >= n * m || s.Size(x + m) > 1 && s[x + m] != s[x])
                          {
                              xmin = Math.Min(xmin, (x % m) * 2);
                              xmax = Math.Max(xmax, (x % m + 1) * 2);
                          }
                          xsum += (x % m) * 2 + 1;
                          sum++;
                          if (s.Size(x - m) > 1 && s[x - m] != s[x])
                          {
                              var res = dfs(x - m);
                              xsum += res.Key;
                              sum += res.Value;
                          }
                      }
                      if (sum * xmin < xsum && xsum < sum * xmax) { }
                      else ok = false;

                      return new KeyValuePair<int, int>(xsum, sum);
                  };

                for (int j = 0; j < m; j++)
                    if (char.IsDigit(t[n - 1][j]))
                    {
                        dfs((n - 1) * m + j);
                        break;
                    }




                if (!ok) IO.Printer.Out.Write("UN");
                IO.Printer.Out.WriteLine("STABLE");
            }
        }

        const long INF = 1L << 60;
        int[] dx = { -1, 0, 1, 0 };
        int[] dy = { 0, 1, 0, -1 };
        //*
        int ri { get { return sc.Integer(); } }
        long rl { get { return sc.Long(); } }
        double rd { get { return sc.Double(); } }
        string rs { get { return sc.Scan(); } }
        //*/

        public IO.StreamScanner sc = new IO.StreamScanner(Console.OpenStandardInput());
        static T[] Enumerate<T>(int n, Func<int, T> f) { var a = new T[n]; for (int i = 0; i < n; ++i) a[i] = f(i); return a; }
        static public void Swap<T>(ref T a, ref T b) { var tmp = a; a = b; b = tmp; }
    }
}

#region main
static class Ex
{
    static public string AsString(this IEnumerable<char> ie) { return new string(ie.ToArray()); }
    static public string AsJoinedString<T>(this IEnumerable<T> ie, string st = " ")
    {
        return string.Join(st, ie.Select(x => x.ToString()).ToArray());
        //return string.Join(st, ie);
    }
    static public void Main()
    {
        var solver = new Program.Solver();
        solver.Solve();
        Program.IO.Printer.Out.Flush();
    }
}
#endregion
#region Ex
namespace Program.IO
{
    using System.IO;
    using System.Text;
    using System.Globalization;
    public class Printer: StreamWriter
    {
        static Printer() { Out = new Printer(Console.OpenStandardOutput()) { AutoFlush = false }; }
        public static Printer Out { get; set; }
        public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
        public Printer(System.IO.Stream stream) : base(stream, new UTF8Encoding(false, true)) { }
        public Printer(System.IO.Stream stream, Encoding encoding) : base(stream, encoding) { }
        public void Write<T>(string format, T[] source) { base.Write(format, source.OfType<object>().ToArray()); }
        public void WriteLine<T>(string format, T[] source) { base.WriteLine(format, source.OfType<object>().ToArray()); }
    }
    public class StreamScanner
    {
        public StreamScanner(Stream stream) { str = stream; }
        public readonly Stream str;
        private readonly byte[] buf = new byte[1024];
        private int len, ptr;
        public bool isEof = false;
        public bool IsEndOfStream { get { return isEof; } }
        private byte read()
        {
            if (isEof) return 0;
            if (ptr >= len) { ptr = 0; if ((len = str.Read(buf, 0, 1024)) <= 0) { isEof = true; return 0; } }
            return buf[ptr++];
        }
        public char Char() { byte b = 0; do b = read(); while ((b < 33 || 126 < b) && !isEof); return (char)b; }

        public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
                sb.Append(b);
            return sb.ToString();
        }
        public string ScanLine()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b != '\n'; b = (char)read())
                if (b == 0) break;
                else if (b != '\r') sb.Append(b);
            return sb.ToString();
        }
        public long Long()
        {
            if (isEof) return long.MinValue;
            long ret = 0; byte b = 0; var ng = false;
            do b = read();
            while (b != 0 && b != '-' && (b < '0' || '9' < b));
            if (b == 0) return long.MinValue;
            if (b == '-') { ng = true; b = read(); }
            for (; true; b = read())
            {
                if (b < '0' || '9' < b)
                    return ng ? -ret : ret;
                else ret = ret * 10 + b - '0';
            }
        }
        public int Integer() { return (isEof) ? int.MinValue : (int)Long(); }
        public double Double() { var s = Scan(); return s != "" ? double.Parse(s, CultureInfo.InvariantCulture) : double.NaN; }
        private T[] enumerate<T>(int n, Func<T> f)
        {
            var a = new T[n];
            for (int i = 0; i < n; ++i) a[i] = f();
            return a;
        }

        public char[] Char(int n) { return enumerate(n, Char); }
        public string[] Scan(int n) { return enumerate(n, Scan); }
        public double[] Double(int n) { return enumerate(n, Double); }
        public int[] Integer(int n) { return enumerate(n, Integer); }
        public long[] Long(int n) { return enumerate(n, Long); }
    }
}
#endregion

#region DisjointSet
public class DisjointSet
{
    int[] par;
    byte[] rank;
    public DisjointSet(int n)
    {
        par = new int[n];
        for (int i = 0; i < n; i++)
            par[i] = -1;
        rank = new byte[n];
    }
    public int this[int id]
    {
        get
        {
            if ((par[id] < 0)) return id;
            return par[id] = this[par[id]];
        }
    }
    public bool Unite(int x, int y)
    {
        x = this[x]; y = this[y];
        if (x == y) return false;
        if (rank[x] < rank[y])
        {
            par[y] += par[x];
            par[x] = y;
        }
        else
        {
            par[x] += par[y];
            par[y] = x;
            if (rank[x] == rank[y])
                rank[x]++;
        }
        return true;
    }
    public int Size(int x) { return -par[this[x]]; }
    public bool IsUnited(int x, int y) { return this[x] == this[y]; }

}
#endregion


Measured memory_bytes: 9352