for(r=readline;;r().length,p()){f=r(n=+r(d=[])).split('');o=[y=i=0];for(z=w=l=f.length;--l;o.push(w-l))d.push(r().split(''));for(z--;i<w;i++){m=i%2;v=m*2-1;for(x=['pop','shift'];d[y];y-=v,o.push(z-=w*v))f.push(d[y][x[m]]());for(y+=v;d[0]&&d[y][0];o.push(z+=v))f.push(d[y][x[m]]());d[x[m]](y-=m^1)};for(;n--;)f.unshift(f.pop());for(i=0;i<o.length;)d[o[i]]=f[i++];for(p=print;d[0];)p(d.splice(0,w).join(''))}
Note that non-ascii characters in the above source code will be escaped (such as \x9f).