#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

#define rd_init() char*rp=({char*mmap();mmap(0l,1l<<25,1,2,0,0ll);})
#define rd() ({int _v=0,_c;while(_c=*rp++-48,_c>=0)_v=_v*10+_c;_v;})
#define wt(v) ({ulong _z=v;do*--wp=_z%10+48;while(_z/=10);})
#define wt1(v) ({char wbuf[64],*wp=wbuf+sizeof wbuf;wt(v);write(1,wp,wbuf+sizeof wbuf-wp);})
#define rep(v,e) for(typeof(e)v=0;v<e;++v)
#define reps(v,s,e) for(typeof(e)v=s;v<e;++v)
#define rreps(v,s,e) for(typeof(e)v=e;--v>=s;)
#define min(a,b) (a<=b?a:b)
#define max(a,b) (a>=b?a:b)
#define chmin(v,a) (v=v<=a?v:a)

char wbuf[6000];
char a[3000];

int main(){
	rd_init();
	int h=rd();
	int w=rd();
	a[0]=1;
	wbuf[0]=*rp;
	rep(i,h+w-2){
		int b=127;
		int s=max(0,i-w-1);
		int e=min(h,i+1);
		reps(y,s,e){
			if(a[y]){
				int x=i-y;
				if(x+1<w){
					chmin(b,rp[y*(w+1)+(x+1)]);
				}
				if(y+1<h){
					chmin(b,rp[(y+1)*(w+1)+x]);
				}
			}
		}
		wbuf[i+1]=b;
		rreps(y,s,e){
			if(a[y]){
				a[y]=0;
				int x=i-y;
				if(x+1<w){
					if(rp[y*(w+1)+(x+1)]==b){
						a[y]=1;
					}
				}
				if(y+1<h){
					if(rp[(y+1)*(w+1)+x]==b){
						a[y+1]=1;
					}
				}
			}
		}
	}
	write(1,wbuf,h+w-1);
	_exit(0);
}