#include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>
#include <fstream>
#include <cassert>
using namespace std;

//common

typedef int  i32;
typedef long long i64,ll;
typedef long double ld;

#define BR "\n"
#define ALL(c) (c).begin(),(c).end()
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define FOR(i,l,r) for(int i=(int)l;i<(int)(r);++i)
#define EACH(it,o) for(auto it = (o).begin(); it != (o).end(); ++it)
#define IN(l,v,r) ((l)<=(v) && (v)<(r))

//config
//#define NDEBUG
//#define INF 1<<30
//#define EPS 1e-8
//const ll MOD =100000007;

//debug
#ifdef NDEBUG
#define DUMP(x)
#define DUMPLN(x)
#define DEBUG(x)
#define DEBUGLN(x)
#define LINE()
#define LINELN()
#define CHECK(exp,act)
#define STOP(e)
#else
#define DUMP(x)  cerr << #x << " = " << (x)
#define DUMPLN(x)  DUMP(x) <<endl
#define DEBUG(x) DUMP(x) << LINE() << " " << __FILE__
#define DEBUGLN(x) DEBUG(x)<<endl
#define LINE()    cerr<< " (L" << __LINE__ << ")"
#define LINELN()    LINE()<<endl
#define CHECK(exp,act)  if(exp!=act){DUMPLN(exp);DEBUGLN(act);}
#define STOP(e)  CHECK(e,true);if(!(e)) exit(1);
#endif

template<class T> inline string toString(const vector<T>& x) {
	stringstream ss;
	REP(i,x.size()){
		if(i!=0)ss<<" ";
		ss<< x[i];
	}
	return ss.str();
}

template<class T> inline string toString(const vector<vector<T> >& map) {
	stringstream ss;
	REP(i,map.size()){
		if(i!=0)ss<<BR;
		ss<< toString(map[i]);
	}
	return ss.str();
}
template<class K,class V>  string toString(map<K,V>& x) {
	string res;stringstream ss;
	for(auto& p:x)ss<< p.first<<":" << p.second<<" ";
	return ss.str();
}

string BITtoString(int bit){
    stringstream ss;
    while(bit!=0){ss<<(bit%2);bit/=2;}
    string res=ss.str();reverse(ALL(res));
    return res;
}

template<typename T,typename V> inline T pmod(T v,V MOD){
	return (v%MOD+MOD)%MOD;
}
#define nextInt(n) scanf("%d",&n)
#define defInt(n) int n;nextInt(n)
#define nextLong(n) scanf("%lld",&n)
#define defLong(n) ll n;nextLong(n)
#define nextDouble(n) scanf("%lf",&n)


int dx[2]={1,0},dy[2]={0,1};
int INF=1<<28;

class Main{
public:

	int dfs(int y1,int x1,int y2,int x2){
		if(!(y1<=y2 && x1<=x2))return 0;
		if(!(IN(0,y1,N) && IN(0,x1,M) && IN(0,y2,N) && IN(0,x2,M))) return -INF;
		if(dp[y1][x1][y2][x2]!=-INF)return dp[y1][x1][y2][x2];

		if(!move[y1][x1][y2][x2])return  dp[y1][x1][y2][x2]=-1;

		int res=-1;
		REP(d1,2)res=max(res,dfs(y1+dy[d1],x1+dx[d1],y2,x2));

		if(islower(board[y1][x1])){
			int c=board[y1][x1]-'a';
			REP(i,holes[c].size()){
				int ny=holes[c][i].first,nx=holes[c][i].second;
				if(!(move[y1][x1][ny][nx] && (y1<=ny && ny <=y2) && (x1<=nx && nx<=x2)))continue;
				REP(d1,2)REP(d2,2){
					res=max(res,dfs(y1+dy[d1],x1+dx[d1],ny-dy[d2],nx-dx[d2])+dfs(ny,nx,y2,x2)+1);
				}
			}
		}
		return dp[y1][x1][y2][x2]=res;
	}

	int N,M;
	vector<string> board;
	vector<vector<vector<vector<int> > > > dp;
	vector<vector<vector<vector<bool> > > > move;
	vector<vector<pair<int,int>>> holes;

	void run(){
		 // ifstream cin("in");
		 // ofstream cout( "out" );
		while(true){
			cin >> N >> M;
			if(N==0)break;
			board=vector<string>(N);
			REP(i,N) cin >> board[i];

			holes=vector<vector<pair<int,int>>> (26);
			REP(y,N)REP(x,M){
				if(isupper(board[y][x]))holes[board[y][x]-'A'].push_back(make_pair(y,x));
			}

			move=vector<vector<vector<vector<bool> > > >(N,vector<vector<vector<bool> > >(M,	vector<vector<bool> >(N,vector<bool>(M))));
			REP(y1,N)REP(x1,M){
				if(board[y1][x1]!='#') move[y1][x1][y1][x1]=true;
				REP(y2,N)REP(x2,M){
					if(y2-1>=y1) move[y1][x1][y2][x2] = move[y1][x1][y2][x2] | move[y1][x1][y2-1][x2];
					if(x2-1>=x1) move[y1][x1][y2][x2] = move[y1][x1][y2][x2] | move[y1][x1][y2][x2-1];
					if(board[y2][x2]=='#') move[y1][x1][y2][x2]=false;
				}
			}

			dp=vector<vector<vector<vector<int> > > >(N,vector<vector<vector<int> > >(M,	vector<vector<int> >(N,vector<int>(M,-INF))));	
			//h=0 w=0
			REP(y,N)REP(x,M)if(board[y][x]!='#') dp[y][x][y][x]=0;
			cout <<dfs(0,0,N-1,M-1)<<endl;
		}
	}
};
int main(){
	cout <<fixed<<setprecision(13);	
	ios::sync_with_stdio(false);
	Main().run();
	return 0;
}

Measured memory_bytes: 67504