#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
#define Inf 1000000001


int main(){
	
	int N,M;
	cin>>N>>M;
	
	vector<int> A(N),B(M);
	rep(i,N){
		cin>>A[i];
	}
	A.insert(A.begin(),0);
	A.push_back(Inf);
	
	vector<vector<int>> bs(A.size());
	
	rep(i,M){
		cin>>B[i];
		int d = distance(A.begin(),lower_bound(A.begin(),A.end(),B[i]));
		bs[d-1].push_back(B[i]);
	}
	
	long long ans = 0;
	
	rep(i,A.size()){
		if(bs[i].size()==0)continue;
		vector<long long> t(1,A[i]);
		rep(j,bs[i].size())t.push_back(bs[i][j]);
		t.push_back(A[i+1]);
		
		long long Ma = 0LL;
		rep(j,t.size()-1){
			Ma = max(Ma,t[j+1] - t[j] - 1);
		}
		ans += A[i+1] - A[i] - 1 - Ma;
	}
	
	cout<<ans<<endl;
	return 0;
}