#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pi pair<int,int>
const int INF = 4e18;
void solve() {
int n,k;
cin>> n>> k;
vector<int> a(n);
for (int i=0; i<n; i++) cin>> a[i];
int l=2, r=a[n-1] - a[0]+2;
int ans = INF;
while (l<=r) {
int mid = (l+r)/2;
int f=a[0], cnt=1, op=0;
for (int i=1; i<n; i++) {
if (a[i]+1 - f > mid) {
cnt++;
op += a[i-1]+1 - f;
f=a[i];
}
}
op += a[n-1]+1-f;
if (cnt <= k) ans = min(ans, op);
//cout<< cnt<< ' '<< op<< ' '<< mid<< '\n';
if (cnt >= k) l=mid+1;
else r=mid-1;
}
cout<< ans<< '\n';
}
int32_t main() {
cin.tie(0)->sync_with_stdio(0);
int t;
t=1;
while (t--) {
solve();
}
}
