#ifdef LOCAL
#include ".debug.hpp"
#else
#define debug(...) 42
#endif
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
using ordered_set = tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>;
signed main() {
#ifndef VOID
cin.tie(nullptr)->sync_with_stdio(false);
#endif
int N, K; cin >> N >> K;
vector<int64_t> A(N); for (auto &a : A) cin >> a;
auto check = [&](const int64_t &target) -> array<int64_t, 2> {
int cnt = 1;
int64_t L = A[0], res = 0;
for (int i = 1; i < N; i++) {
if (A[i] - L + 1 <= target) continue;
else res += (A[i - 1] - L + 1), cnt++, L = A[i];
}
res += A.back() - L + 1;
return {cnt <= K, res};
};
int64_t low = 1, high = INT_MAX, mid, best = INT_MAX;
while (low <= high) {
mid = (low + high) >> 1;
array<int64_t, 2> res = check(mid);
debug(mid); debug(res);
if (res[0]) best = res[1], high = mid - 1;
else low = mid + 1;
}
cout << best;
return 0;
}
