#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("inline")
#include<bits/stdc++.h>
using namespace std;
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
int As;
int Bs;
int A[5000];
int B[5000];
int main(){
  int i;
  int x = 0;
  int y = 0;
  rd(As);
  rd(Bs);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(As);Lj4PdHRW++){
      rd(A[Lj4PdHRW]);
    }
  }
  {
    int e98WHCEY;
    for(e98WHCEY=(0);e98WHCEY<(Bs);e98WHCEY++){
      rd(B[e98WHCEY]);
    }
  }
  for(i=(0);i<(As*Bs);i++){
    if(A[x] == B[y]){
      wt_L(i+1);
      wt_L('\n');
      return 0;
    }
    x++;
    y++;
    if(x==As){
      x = 0;
    }
    if(y==Bs){
      y = 0;
    }
  }
  wt_L(-1);
  wt_L('\n');
  return 0;
}
// cLay version 20210611-1 [beta]

// --- original code ---
// int As, Bs, A[5000], B[5000];
// {
//   int x = 0, y = 0;
//   rd(As, Bs, A(As), B(Bs));
// 
//   rep(i,As*Bs){
//     if(A[x] == B[y]) wt(i+1), return 0;
//     x++; y++;
//     if(x==As) x = 0;
//     if(y==Bs) y = 0;
//   }
//   wt(-1);
// }