#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = (l); i < (r); i++)
using namespace std;

typedef long long ll;

int main() {
    int N, M;
    cin >> N >> M;
    vector<int> A(N), B(M);
    rep(i, 0, N) cin >> A[i];
    rep(i, 0, M) cin >> B[i];
    ll ans = 0;
    if (B[0] < A[0]) ans = A[0] - B[0];
    if (B[M - 1] > A[N - 1]) ans += B[M - 1] - A[N - 1];
    rep(i, 1, N) {
        int l = upper_bound(B.begin(), B.end(), A[i - 1]) - B.begin();
        int r = upper_bound(B.begin(), B.end(), A[i]) - B.begin() - 1;
        if (l > r) continue;
        int d = max(B[l] - A[i - 1] - 1, A[i] - B[r] - 1);
        for (int j = l; j < r; j++) {
            d = max(d, B[j + 1] - B[j] - 1);
        }
        //cout << i << " " << d << endl;
        ans += A[i] - A[i - 1] - 1 - d;
    }
    cout << ans << endl;
}