#include <cstdio>
#include <cctype>
#include <vector>
using namespace std;

typedef int State; // [mask:8][y:5][x:5]

int solve( int W, int H, int start, int goal, char M[] )
{
	bool visited[1<<18] = {0};
	vector<State> Q(1, start);
	for(int steps=0; !Q.empty(); ++steps)
	{
		vector<State> Q2; Q2.swap(Q);
		for each(State s in Q2)
			if( !visited[s] )
			{
				visited[s] = true;
				if( s%1024 == goal ) return steps;

				char c = M[s%1024];
				if( c=='#' || isupper(c) && !(s&(1<<10+c-'A')) ) continue;

				for(int j=0; j<4; ++j)
				{
					static int dir[] = {-1,+1,-32,+32};
					char cc = M[s%1024+dir[j]];
					Q.push_back( islower(cc) ? s+dir[j] ^ (1<<10+cc-'a') : s+dir[j] );
				}
			}
	}
	return -1;
}

int main()
{
	for(int W,H; scanf("%d %d\n",&W,&H),W|H; )
	{
		// input ----------------------------------------------
		char M[32*32];
		for(int y=0; y<H; ++y)
			gets(&M[y*32]);

		// manipulate -----------------------------------------
		State start, goal;

		bool has[256]={0};
		for(int y=0; y<H; ++y)
		for(int x=0; x<W; ++x)
			has[M[y*32+x]] = true;

		char cvt[26]={0}, alp=0;
		for(int y=0; y<H; ++y)
		for(int x=0; x<W; ++x)
		{
			int p = y*32+x;
			     if( M[p]=='@' ) start = p;
			else if( M[p]=='<' ) goal  = p;
			else if( islower(M[p]) && has[M[p]-'a'+'A'] )
			{
				if( !cvt[M[p]-'a'] )
					cvt[M[p]-'a'] = ++alp;
				M[p] = cvt[M[p]-'a']-1+'a';
			}
			else if( isupper(M[p]) && has[M[p]-'A'+'a'] )
			{
				if( !cvt[M[p]-'A'] )
					cvt[M[p]-'A'] = ++alp;
				M[p] = cvt[M[p]-'A']-1+'A';
			}
			else if( islower(M[p]) ) M[p] = '.';
			else if( isupper(M[p]) ) M[p] = '#';
		}

		// solve ----------------------------------------------
		printf("%d\n", solve(W,H,start,goal,M) );
	}
}