#include <bits/stdc++.h>
using namespace std;
#define int long long int
using pi = pair<int, int>;
const int INF = 4e18;
signed main() { // is this alien trick ??
cin.tie(0)->sync_with_stdio(0);
int n, k; cin >> n >> k;
vector<int> times(n);
for (int i = 0; i < n; i++) {
cin >> times[i];
times[i]--;
}
auto g = [&](int la) {
vector<int> DP(n, INF), cnt(n, -INF), prefix_min(n+1, INF);
DP[0] = 1+la; cnt[0] = 1;
int pb = 0, pc = 0, pv = -times[0] + 1 + la;
for (int i = 1; i < n; i++) {
int cand = DP[i-1] - times[i] + 1 + la;
if (cand < pv || cand == pv && cnt[i-1] < pc) {
pc = cnt[i-1];
pv = cand;
pb = i;
}
DP[i] = times[i] + pv;
cnt[i] = pc+1;
}
return make_pair(DP[n-1], cnt[n-1]);
};
int lo = 0, hi = 1e8;
while (lo < hi) {
int mid = lo + (hi - lo) / 2;
pi fx = g(mid);
if (fx.second > k) {
lo = mid + 1;
} else {
hi = mid;
}
}
pi r1 = g(lo), r2 = g(lo-1);
cout << max(r1.first - k * lo, r2.first - k * (lo-1)) << "\n";
return 0;
}
