#include <bits/stdc++.h>
using namespace std;
#define fast_io ios::sync_with_stdio(false); cin.tie(nullptr);
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
typedef long long ll;
typedef long double ld;
#define chmin(a,b) a = min(a,b);
#define chmax(a,b) a = max(a,b);
#define bit_count(x) __builtin_popcountll(x)
#define leading_zero_count(x) __builtin_clz(x)
#define trailing_zero_count(x) __builtin_ctz(x)
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) a / gcd(a,b) * b
#define rep(i,n) for(int i = 0 ; i < n ; i++)
#define rrep(i,a,b) for(int i = a ; i < b ; i++)
#define repi(it,S) for(auto it = S.begin() ; it != S.end() ; it++)
#define pt(a) cout << a << endl;
#define debug(a) cout << #a << " " << a << endl;
#define all(a) a.begin(), a.end()
#define endl "\n";
#define v1(n,a) vector<ll>(n,a)
#define v2(n,m,a) vector<vector<ll>>(n,v1(m,a))
#define v3(n,m,k,a) vector<vector<vector<ll>>>(n,v2(m,k,a))
#define v4(n,m,k,l,a) vector<vector<vector<vector<ll>>>>(n,v3(m,k,l,a))
template<typename T,typename U>istream &operator>>(istream&is,pair<T,U>&p){is>>p.first>>p.second;return is;}
template<typename T,typename U>ostream &operator<<(ostream&os,const pair<T,U>&p){os<<p.first<<" "<<p.second;return os;}
template<typename T>istream &operator>>(istream&is,vector<T>&v){for(T &in:v){is>>in;}return is;}
template<typename T>ostream &operator<<(ostream&os,const vector<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}
template<typename T>istream &operator>>(istream&is,vector<vector<T>>&v){for(T &in:v){is>>in;}return is;}
template<typename T>ostream &operator<<(ostream&os,const vector<vector<T>>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?"\n":"");}return os;}
template<typename T>ostream &operator<<(ostream&os,const set<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}
template<typename T>ostream &operator<<(ostream&os,const multiset<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}

template<typename T = long long>
struct SegmentTree{
    int n , n_;
    vector<T> dat;
    vector<pair<int,int>> range;
    SegmentTree(int _n){
        n = 1 ;
        n_ = _n ;
        while(n < _n) n *= 2;
        dat.resize(2 * n - 1,0);
        range.resize(2 * n - 1);
        range[0] = {0,n};
        for(int i = 0 ; i < n - 1 ; i++){
            auto [l,r] = range[i];
            range[2*i+1] = {l,l+(r-l)/2};
            range[2*i+2] = {l+(r-l)/2,r};
        }
    }
    void add(int k , T x){
        k += n - 1 ;
        dat[k] += x ;
        while(k > 0){
            k = (k - 1) / 2 ;
            dat[k] = dat[2*k+1] + dat[2*k+2] ;
        }
    }
    T sub_query(int a , int b , int k , int l , int r){
        if(r <= a || b <= l) return 0 ;
        if(a <= l && r <= b) return dat[k] ;
        T lef = sub_query(a,b,2*k+1,l,(l+r)/2) ;
        T rig = sub_query(a,b,2*k+2,(l+r)/2,r) ;
        return lef + rig ;
    }
    T query(int a , int b){
        return sub_query(a,b,0,0,n) ;
    }
};


void solve(){
    ll n, m;
    cin >> n >> m;
    vector<ll> P(n);
    cin >> P;
    rep(i,n) P[i]--;
    ll sum = 0;
    SegmentTree<ll> segtree(n);
    rep(i,n){
        sum += i - segtree.query(0,P[i]);
        segtree.add(P[i],1);
    }
    if(sum == 0){
        pt(0)
        return;
    }
    if(m % 2 == 0){
        if(sum % 2 != m % 2){
            pt(-1)
            return;
        }
        if(m > sum){
            pt(m)
            return;
        }
        pt((sum / m + (sum % m != 0)) * m)
        return;
    }
    if(m % 2 != sum % 2) m *= 2;
    if(m > sum){
        pt(m)
        return;
    }
    pt((sum / m + (sum % m != 0)) * m)
}

int main(){
    fast_io
    int t = 1;
    // cin >> t;
    rep(i,t) solve();
}