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

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

short d[4096];

main(){
	char*rp=mmap(0l,1l<<25,1,2,0,0ll);
	RD(h);
	RD(w);++w;
	long i=0,z=0,a=0,b=0,c=0;
	while(c=*rp++){
		if(c&1){
			c=b;
			b=d[i];
			a=(a<b?a<c?a:c:b<c?b:c)+1;
			z=z>=a?z:a;
		}else{
			a=0;
		}
		d[i+w&4095]=a;
		i=i+1&4095;
	}
	z=z+1>>1;
	{
		long n=0;
		long d=0;
		while(++n,d=d<<8|0x30|z%10,z/=10);
		write(1,&d,n);
	}
	_exit(0);
}