// Source: https://usaco.guide/general/io

#include <bits/stdc++.h>
using namespace std;
#define int long long
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")


signed main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);
    int n,x;cin>>n>>x;
    vector<int> v(n);
    for(int i = 0;i<n;i++){
        cin>>v[i];
    }
    int mn=1e15;
    for(int i = 0;i<=n-x;i++){
        int cost=0;
        int currh=v[i];
        //cout<<i<<':'<<endl;
        for(int j = i+1;j<i+x;j++){
            if(v[j]==currh-1){
                currh--;
            }
            else if(v[j]<currh-1){
                currh--;
                cost+=currh-v[j];
            }
            else{
                int diff=v[j]-(currh-1);
                cost+=diff*(j-i);
                currh=v[j];
            }
            //cout<<j<<' '<<cost<<endl;
        }
        mn=min(mn,cost);
    }
    cout<<mn<<endl;
}