import java.io.*;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int q = sc.nextInt();
        Map<Integer, List<Integer>> places = new HashMap<>();
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            if (!places.containsKey(x)) {
                places.put(x, new ArrayList<>());
            }
            places.get(x).add(i);
        }
        int[] costs = new int[n];
        Arrays.fill(costs, Integer.MAX_VALUE);
        PriorityQueue<Path> queue = new PriorityQueue<>();
        queue.add(new Path(0, 0));
        while (q-- > 0) {
            while (queue.size() > 0) {
                Path p = queue.poll();
                if (costs[p.idx] <= p.value) {
                    continue;
                }
                costs[p.idx] = p.value;
                if (p.idx > 0) {
                    queue.add(new Path(p.idx - 1, p.value + 1));
                }
                if (p.idx < n - 1) {
                    queue.add(new Path(p.idx + 1, p.value + 1));
                }
            }
            int x = sc.nextInt();
            for (int y : places.get(x)) {
                queue.add(new Path(y, costs[y]));
            }
            Arrays.fill(costs, Integer.MAX_VALUE);
        }
        System.out.println(queue.poll().value);
    }
    
    static class Path implements Comparable<Path> {
        int idx;
        int value;
        
        public Path(int idx, int value) {
            this.idx = idx;
            this.value = value;
        }
        
        public int compareTo(Path another) {
            return value - another.value;
        }
    }
}
class Scanner {
    BufferedReader br;
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
        } catch (Exception e) {
            
        }
    }
    
    public int nextInt() {
        return Integer.parseInt(next());
    }
    
    public long nextLong() {
        return Long.parseLong(next());
    }
    
    public double nextDouble() {
        return Double.parseDouble(next());
    }
    
    public String next() {
        try {
            while (!st.hasMoreTokens()) {
                st = new StringTokenizer(br.readLine());
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            return st.nextToken();
        }
    }
    
}