char z[6000],s[3000][3001];
ll@h,@w;
rd(s(h,w));
z[0]=s[0][0];
rep(i,1,h+w-1){
	z[i]=127;
	rep(j,h){
		ll k=i-j;
		if(k>=0&&k<w){
			if(j&&s[j-1][k]==z[i-1]||k&&s[j][k-1]==z[i-1]){
				z[i]<?=s[j][k];
			}else{
				s[j][k]=0;
			}
		}
	}
}
wt(z);