#include <iostream>
#include <vector>
using namespace std;

typedef long long ll;
const ll INF = 1e18;
int main(){
    ios_base::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;

    vector<int> v(n);
    for(int i = 0; i < n; i++) {
        cin >> v[i];
    }

    ll till_now = 0;
    ll res = INF;
    for(int i = 0; i < n; i++) {
        ll x = 0;
        vector<int> v2 = v;
        bool ok = true;
        if(i + k - 1 < n) {
            for(int j = i + 1; j < i + k and j < n; j++) {
                if(v2[j] > v2[j - 1]) {
                    ok = false;
                    break;
                }
                if(v2[j - 1] - v2[j] != 1) {
                    x += abs(v2[j] - v2[j - 1] + 1) * 5;
                    v2[j] = v2[j - 1] - 1;
                    if(v2[j] < 1) {
                        ok = false;
                        break;
                    }
                }
            }

            if(ok) {
                res = min(res, x + till_now);
            }
        }

        till_now += 10;
    }

    if(res == INF) {
        res = -1;
    }
    cout << res << endl;

    return 0;
}