#include<bits/stdc++.h>
using namespace std;
long long n, m, ans, a[200005], s[200005], l;
signed main() {
//	freopen("chess.in", "r", stdin);
//	freopen("chess.out", "w", stdout);
	cin >> n >> l >> m;
	for(int i = 1; i <= m; i++) cin >> s[i];
	sort(s + 1, s + m + 1);
	for(int i = 1;i < m;i++) a[i] = s[i + 1] - s[i];
	sort(a + 1, a + m);
	for(int i = 1;i <= m - n;i++) ans += a[i];
	ans = ans + n;
	cout << ans;
	return 0;
}