//AzaLE (Azamat Alisherov)
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
using namespace std;
using namespace __gnu_pbds;
#pragma GCC optimize("Ofast,unroll-loops")
#define size(x) (int)x.size()
#define int long long
//#define endl '\n'
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define F first
#define S second
template<class T> using oset = tree<T, null_type, less_equal<T>,
rb_tree_tag, tree_order_statistics_node_update>;
const int MOD = 1e9 + 7;// 119 * (1 << 23) + 1;
int binpow (int a, int n){
int res = 1;
while (n){
if (n & 1){
res = (res * a) % MOD;
}
a = (a * a) % MOD;
n >>= 1;
}
return res % MOD;
}
vector <int> v;
int n, k;
int check(int x){
int l = 0;
int ret = 0;
int curcord = v[l];
int cnt = 0;
for(int r = 0; r < n; r++){
if(v[r + 1] - curcord + 1 > x){
ret += v[r] - curcord + 1;
curcord = v[r + 1];
cnt++;
}
}
return (cnt <= k ? ret : -1);
}
void chomp(){
cin >> n >> k;
if(n == 1){
cout << 1;
return;
}
v.resize(n);
for(int i = 0; i < n; i++){
cin >> v[i];
}
v.push_back(1e18);
int l = 0, r = 1e10;
int ans = 1e10;
while(l <= r){
int mid = (l + r) / 2;
int res = check(mid);
//cout << mid << " " << res << endl;
if(res != -1){
ans = min(ans, res);
r = mid - 1;
}
else{
l = mid + 1;
}
}
cout << ans;
}
signed main(){
//ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
int tasks = 1;
//cin >> tasks;
while(tasks--){
chomp();
}
}
/*
*/
