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

char*mmap();
#define RD(v) int v=0;{int _c;while(_c=*rp++-48,_c>=0)v=v*10+_c;}

d[3001*3000];

main(){
	char*rp=mmap(0l,1l<<28,1,2,0,0ll);
	RD(h);
	RD(w);++w;
	int z=0,i;
	for(i=0;i<=w;++i){
		z|=d[i]=*rp++&1;
	}
	int a=0,b=d[0],c;
	while(c=*rp++){
		if(c&1){
			c=b;
			b=d[i-w];
			a=(a<b?a<c?a:c:b<c?b:c)+1;
			z=z>=a?z:a;
		}else{
			a=0;
		}
		d[i]=a;
		++i;
	}
	z=z+1>>1;
	{
		int n=0;
		long d=0;
		while(++n,d=d<<8|0x30|z%10,z/=10);
		write(1,&d,n);
	}
	_exit(0);
}