#include <bits/stdc++.h>

using namespace std;

int arr[100100];
int L, n, m;

int check(int mid) {
    int cnt = 0, posi = 0;
    for (int i = 1; i <= n; i++) {
        if (arr[i] < posi + mid)
            cnt++;
        else
            posi = arr[i];
    }

    if (cnt > m)
        return 0;
    else
        return 1;
}

int main() {
    cin >> L >> n >> m;

    for (int i = 1; i <= n; i++) {
        cin >> arr[i];
    }

    int l = 0, r = L;
    while (l < r) {
        int mid = (l + r + 1) / 2;
        if (check(mid))
            l = mid;
        else
            r = mid - 1;
    }

    cout << l;

    system("pause");
    return 0;
}