#include <bits/stdc++.h>
#define boAshraf ios_base::sync_with_stdio(false); cin.tie(NULL);
#define ll long long
#define sz(s) (int)(s).size()
#define all(s) (s).begin(),(s).end()
using namespace std;
void File();
void sol();
int main() {
    boAshraf
    File();
    int t = 1;
//    cin >> t;
    while (t--) {
        sol();
    }
    return 0;
}

void sol() {
    int n,k;
    cin >> n >> k;
    vector<int>a(n);
    for(auto &it:a)cin>>it;
    if(k <= 2)return void(cout<<0);

    vector<int>pre(n, 1), suff(n, 1);
    vector<int>difL(n), difR(n);
    {
        int mx = 1;

        for (int i = 1; i < n; i++) {
            if (i >= 2 && (a[i] - a[i - 1]) == (a[i - 1] - a[i - 2])) pre[i] = pre[i - 1] + 1; else pre[i] = 2;
            difL[i] = a[i] - a[i - 1];
            mx = max(mx, pre[i]);
        }
        for (int i = n - 2; i >= 0; i--) {
            if (i + 2 < n && (a[i + 1] - a[i]) == (a[i + 2] - a[i + 1])) suff[i] = suff[i + 1] + 1; else suff[i] = 2;
            difR[i] = a[i + 1] - a[i];
            mx = max(mx, suff[i]);
        }
        if (mx >= k) return void(cout<<0);
    }

    vector<int> vals=a;
    sort(all(vals));
    vals.erase(unique(all(vals)),vals.end());

    auto get_idx=[&](int x){
        auto it=lower_bound(all(vals), x);
        if(it==vals.end()||*it!=x) return -1;
        return (int)(it-vals.begin());
    };

    vector<vector<int>> idxs(sz(vals));
    for(int i=0; i < n; i++){
        int id=get_idx(a[i]);
        assert(~id);
        idxs[id].push_back(i);
    }

    map<pair<int,int>, pair<int,int>> best;
    int ans=1e9;

    for(int r=0; r < n; r++){
        if(suff[r] >= 2){
            int d=difR[r], needval= a[r] - d;
            int rem= k - suff[r];
            if(rem == 1){
                int id=get_idx(needval);
                if(~id){
                    auto &v=idxs[id];
                    auto p=lower_bound(all(v), r);
                    if(p!=v.begin()){
                        int l=*prev(p);
                        ans=min(ans, r - l - 1);
                    }
                }
            }
            else{
                auto it=best.find({needval, d});
                if(it != best.end()){
                    auto [mx,l]=it->second;
                    if(mx >= rem && l < r) ans=min(ans, r - l - 1);
                }
            }
        }
        if(pre[r] >= 2){
            pair<int,int> key={a[r], difL[r]};
            auto it=best.find(key);
            if(it == best.end() || pre[r] > it->second.first || (pre[r] == it->second.first && r > it->second.second))
                best[key]={pre[r], r};
        }
    }

    for(int l=0; l < n; l++) {
        if (pre[l] >= k - 1) {
            int t = a[l] + difL[l];
            int id = get_idx(t);
            if (~id) {
                auto &it = idxs[id];
                auto p = upper_bound(all(it), l);
                if (p != it.end()) {
                    int r = *p;
                    ans = min(ans, r - l - 1);
                }
            }
        }
    }

    cout<<(ans==1e9?-1:ans);
}

void File() {
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
}