#line 1 "main.cpp"
#include <bits/stdc++.h>
#include <atcoder/all>
// #include "library/templates/template.cpp"
using namespace atcoder;
using namespace std;
using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

long long solve(int N, int M, const std::vector<long long> &A, const std::vector<long long> &B)
{
    // ... solve
    ll ans = 0;
    int j = 0;
    if (A[0] > B[0])
    {
        ans += A[0] - B[0];
        while (j < M && B[j] < A[0])
        {
            j++;
        }
    }
    for (int i = 0; i + 1 < N; i++)
    {
        // A[i]...A[i+1]の間にあるB
        vector<ll> D;
        D.push_back(A[i]);
        while (j < M && B[j] < A[i + 1])
        {
            D.push_back(B[j]);
            j++;
        }
        D.push_back(A[i + 1]);
        if (D.size() > 2)
        {
            ll mindiff = 0;
            for (int t = 0; t + 1 < D.size(); t++)
            {
                mindiff = max(mindiff, D[t + 1] - D[t] - 1);
            }

            ans += A[i + 1] - A[i] - 1 - mindiff;
        }
    }
    if (A[N - 1] < B[M - 1])
    {
        ans += B[M - 1] - A[N - 1];
    }
    return ans;
}

int main()
{
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    int N, M;
    std::cin >> N;
    std::vector<long long> A(N);
    std::cin >> M;
    std::vector<long long> B(M);
    for (int i = 0; i < N; ++i)
    {
        std::cin >> A[i];
    }
    for (int i = 0; i < M; ++i)
    {
        std::cin >> B[i];
    }
    auto ans = solve(N, M, A, B);
    std::cout << ans << '\n';
    return 0;
}