#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define vi vector<long long>
#define all(x) x.begin(), x.end()
#define endl "\n"
void solution();
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
solution();
return 0;
}
const int N = 1e5+10;
ll a[N];
ll n, k;
ll cost(int i, int j) {
return a[j]+1 - a[i];
}
ll isValid(ll c) {
ll ans = 0;
int z = 0;
for (int i = 1; i <= n;) {
int j = i;
while (j <= n && cost(i, j) <= c) {
j++;
}
z++;
ans += cost(i, j-1);
i = j;
}
return (z <= k ? ans : 0);
}
void solution() {
cin >> n >> k;
for (int i = 1; i <= n; i++) cin >> a[i];
ll p = 1, q = 1e18;
ll ans = 1e18;
while (p <= q) {
ll mid = (p + q)/2;
ll temp = isValid(mid);
if (temp) {
ans = temp;
q = mid-1;
} else p = mid+1;
}
cout << ans << endl;
}
