#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;}
#define wt(v) {long _z=v;do*--wp=_z%10+48;while(_z/=10);}

int a[5000],b[5000];

main(){
	char*rp=mmap(0l,1l<<25,1,2,0,0ll);
	rd(n);
	rd(m);
	for(int i=0;i<n;++i){
		rd(ai);
		a[i]=ai;
	}
	for(int i=0;i<m;++i){
		rd(bi);
		b[i]=bi;
	}
	int i=0,ia=0,ib=0;
	for(;;){
		++i;
		if(a[ia]==b[ib]){
			goto found;
		}
		++ia; ia=ia==n?0:ia;
		++ib; ib=ib==m?0:ib;
		if(!(ia|ib)){
			break;
		}
	}
	write(1,"-1",2);
	_exit(0);
	
	found:;
	char wbuf[64],*wp=wbuf+sizeof wbuf;
	wt(i);
	write(1,wp,wbuf+sizeof wbuf-wp);
	_exit(0);
}