int arr[MAXN];
int vis[MAXN];
 
int L, N, M;
bool Check(int num)
{
    int cur = 0, cnt = 0;
    for (int i = 1; i < N; i++)
    {
        if (arr[i] - arr[cur] < num)
        {
            cnt++;
            if (cnt > M) return false;
        }
        else cur = i;
    }
    return true;
}
 
int main()
{
    //ROP;
    scanf("%d%d%d", &L, &N, &M);
    for (int i = 0; i < N; i++) scanf("%d", &arr[i]);
    arr[N] = 0; arr[N+1] = L;
    N += 2;
    sort(arr, arr+N);
    int l = 0, r = L, ans = L;
    while (l <= r)
    {
        int mid = MID(l, r);
        if (Check(mid))
        {
            ans = mid;
            l = mid+1;
        }
        else r = mid-1;
    }
    printf("%d\n", ans);
    return 0;
}