#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
typedef long long ll;
using namespace __gnu_pbds;
using namespace std;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n, k;
cin >> n >> k;
vector<ll> T(n+1);
for(int i=1; i<=n; ++i) cin >> T[i];
ll inf = 1e15;
vector<pair<ll, ll>> dp(n+1, pair<ll, ll>({inf, 0}));
ll start = -inf;
ll koniec = inf;
ll wynik = 0;
while(start+1<koniec) {
int alpha = (start+koniec)/2;
dp[1] = {1-alpha, 1};
for(int i=2; i<=n; ++i) {
int d = T[i]-T[i-1];
dp[i] = dp[i-1];
if(dp[i].first+1-alpha < dp[i].first + d) {
dp[i].first += 1-alpha;
dp[i].second++;
} else {
dp[i].first += d;
}
}
int z = dp[n].second;
wynik = dp[n].first + z*alpha;
//cout << z << "\n";
//for(auto[a, b] : dp) cout << a << " " << b << "\n";
if(z==k) break;
if(z > k) koniec = alpha;
else start = alpha;
}
cout << wynik << "\n";
return 0;
}
