#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <cctype>
#include <cassert>
using namespace std;

typedef pair<int,int>    Pos; // (y,x)
typedef int             Mask; // |A-Z| bits. 0: close, 1: open
typedef pair<Pos,Mask> State;

// solve ---------------------------------------------------------

int bfs( State start, Pos goal, const vector<string>& mp )
{
	set<State> visited;

	vector<State> Q(1, start);
	for(int steps=0; !Q.empty(); ++steps)
	{
		vector<State> nQ;
		for(int i=0; i!=Q.size(); ++i)
		{
			State s = Q[i];
			if( visited.count(s) ) continue;
			visited.insert(s);

			if( s.first == goal ) return steps;

			char c = mp[s.first.first][s.first.second];
			if( c=='#' )                                 continue;
			if( isupper(c) && !(s.second&(1<<(c-'A'))) ) continue;

			for(int dir=0; dir!=4; ++dir)
			{
				static int dy[] = {-1,+1,0,0};
				static int dx[] = {0,0,-1,+1};
				int  y = s.first.first  + dy[dir];
				int  x = s.first.second + dx[dir];
				char c = mp[y][x];
				Mask m = islower(c) ? s.second ^ (1<<(c-'a')) : s.second;

				nQ.push_back(State(Pos(y,x),m));
			}
		}
		Q.swap(nQ);
	}

	return -1;
}

int solve(const vector<string>& mp)
{
	// スタートセルとゴールセルを見つける
	Pos start, goal;
	for(int y=0; y!=mp.size(); ++y)
		for(int x=0; x!=mp[y].size(); ++x)
			if( mp[y][x]=='@' )
				start = Pos(y,x);
			else if( mp[y][x]=='<' )
				goal = Pos(y,x);

	// BFSで最短パスを探す
	return bfs( State(start,0), goal, mp );
}

// read the input --------------------------------------------------

int main()
{
	int W, H;
	while(cin >> W >> H, W|H)
	{
		assert( 3<=W&&W<=30 );
		assert( 3<=H&&H<=30 );

		vector<string> mp(H);
		for(int h=0; h!=H; ++h)
			cin >> mp[h];

		cout << solve(mp) << endl;
	}
}