#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cctype>
#include <cstring>
#include <string>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <numeric>
#include <complex>
#include <utility>
#include <iomanip>
#include <algorithm>
#include <functional>
using namespace std;

typedef vector<int> vint;
typedef vector<vint> vvint;
typedef vector<string> vst;
typedef pair<int,int> pint;
typedef long long ll;

#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define all(s) (s).begin(),(s).end()
#define each(i,s) for (typeof((s).begin()) i = (s).begin(); i != (s).end(); ++i)
#define debug(x) cout<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<<endl
template<class T1, class T2> ostream& operator<<(ostream &s, pair<T1,T2> P){return s<<'<'<<P.first<<","<<P.second<<'>';}
template<class T> ostream& operator<<(ostream &s, vector<T> P) {s<<"{ ";for(int i=0;i<P.size();++i){if(i>0)s<<", ";s<<P[i];}return s<<" }"<<endl;}
template<class T1, class T2> ostream& operator<<(ostream &s, map<T1,T2> P) {s<<"{ ";for(typeof(P.begin()) it=P.begin();it!=P.end();++it){if(it!=P.begin())s<<", ";s<<'<'<<it->first<<"->"<<it->second<<'>';}return s<<" }"<<endl;}
template<class T1, class T2> pair<T1,T2> operator+(pair<T1,T2> P, pair<T1,T2> Q) {return make_pair(P.first+Q.first,P.second+Q.second);}
template<class T1, class T2> pair<T1,T2> operator-(pair<T1,T2> P, pair<T1,T2> Q) {return make_pair(P.first-Q.first,P.second-Q.second);}
template<class T> pair<T,T> operator*(pair<T,T> P, int a) {return make_pair(P.first*a,P.second*a);}
template<class T> pair<T,T> operator*(pair<T,T> P, pair<T,T> Q) {return make_pair(P.first*Q.first-P.second*Q.second,P.first*Q.second+P.second*Q.first);}




pint dr[2] = {mp(1,0), mp(0,1)};
int H,W;
vst str;

vector<vector<pint> > hset;
bool reach[52][52][52][52];
int dp[52][52][52][52];

int solve() {
    hset.clear();
    hset.resize(26);
    memset(reach, 0, sizeof(reach));
    for (int i = 0; i < str.size(); ++i) {
        for (int j = 0; j < str[0].size(); ++j) {
            if (str[i][j] == '#') continue;
            if (str[i][j] >= 'A' && str[i][j] <= 'Z') hset[(int)(str[i][j]-'A')].pb(mp(i,j));
            reach[i][j][i][j] = true;
            queue<pint> que;
            que.push(mp(i,j));
            while (!que.empty()) {
                pint p = que.front();
                que.pop();
                for (int o = 0; o < 2; ++o) {
                    pint np = p + dr[o];
                    if (np.fi >= 0 && np.fi < str.size() && np.se >= 0 && np.se < str[0].size()) {
                        if (!reach[i][j][np.fi][np.se] && str[np.fi][np.se] != '#') {
                            reach[i][j][np.fi][np.se] = true;
                            que.push(np);
                        }
                    }
                }
            }
        }
    }
    if (!reach[0][0][str.size()-1][str[0].size()-1]) return -1;
    
    memset(dp, 0, sizeof(dp));
    for (int i = 1; i <= str.size(); ++i) {
        for (int j = 1; j <= str[0].size(); ++j) {
            for (int k = 0; k+i <= str.size(); ++k) {
                for (int l = 0; l+j <= str[0].size(); ++l) {
                    if (!reach[k][l][k+i-1][l+j-1]) continue;
                    int temp = 0;
                    
                    if (reach[k+1][l][k+i-1][l+j-1]) temp = max(temp, dp[k+1][l][k+i][l+j]);
                    if (reach[k][l+1][k+i-1][l+j-1]) temp = max(temp, dp[k][l+1][k+i][l+j]);
                    
                    if (str[k][l] >= 'a' && str[k][l] <= 'z') {
                        for (int x = 0; x < hset[(int)(str[k][l]-'a')].size(); ++x) {
                            int p = hset[(int)(str[k][l]-'a')][x].fi;
                            int q = hset[(int)(str[k][l]-'a')][x].se;
                            
                            if (p >= k && p < k+i && q >= l && q < l+j && reach[k][l][p][q] && reach[p][q][k+i-1][l+j-1]) {
                                int temp1 = 0, temp2 = 0;
                                
                                temp1 = max(temp1, dp[k+1][l][p+1][q]+1);
                                temp1 = max(temp1, dp[k+1][l][p][q+1]+1);
                                temp1 = max(temp1, dp[k][l+1][p+1][q]+1);
                                temp1 = max(temp1, dp[k][l+1][p][q+1]+1);
                            
                                temp2 = max(temp2, dp[p][q][k+i][l+j]);
                                
                                temp = max(temp, temp1+temp2);
                            }
                        }
                    }
                    
                    //if (temp > 1) cout << k << ", " << l << ", " << k+i-1 << ", " << l+j-1 << " : " << temp << endl;
                    
                    dp[k][l][k+i][l+j] = temp;
                }
            }
        }
    }
   
    return dp[0][0][str.size()][str[0].size()];                            
}

int main() {
    //freopen( "/Users/macuser/Documents/Programming/Contest/JAG Practice Asia 2012 E - Stack Maze.in", "r", stdin );
    
    while (cin >> H >> W) {
        if (H == 0 && W == 0) break;
        str.clear();
        for (int i = 0; i < H; ++i) {
            string s;
            cin >> s;
            str.pb(s);
        }
        cout << solve() << endl;
    }
    
    return 0;
}


Measured memory_bytes: 36956