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