#include <iostream>
#include <algorithm>
typedef long long ll;
using namespace std;
const int N = 1e5 + 10;
ll a[N], k, n, m;
ll rec[N];
bool check(int mid)
{
    ll start = 0, count = 0;
    for (int i = 1; i <= n; i++)
    {
        if (a[i] - start < mid) count++;
        else start = a[i];
    }
    return count <= m;
}

ll find(ll l, ll r)
{
    while (l < r)
    {
        ll mid = r + l + 1 >> 1;
        if (check(mid))
            l = mid;
        else
            r = mid - 1;
    }
    return l;
}                                                                                                                                                           

int main()
{
    cin >> k >> n >> m;
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    a[n + 1] = k;
    cout << find(0, k);
    return 0;
}