// あの花はまだどこかに咲いているに違いない
// 我们会慢慢长大成人，在不断流逝的季节之中，盛开在路旁的花儿们，也在渐渐变化，盛开在那个季节里的花朵叫什么名字呢，在风中小小摇曳，伸手触碰便被扎得疼痛，把脸凑近，就能闻到一股淡淡的青涩的太阳芳香，那股芳香渐渐消散，而我们渐渐长大，但是，那朵花一定依旧盛开在某处。
 
// Code by ttq012.
#include <bits/stdc++.h>
#define eb emplace_back
#define int long long
using namespace std;
const int N = 500100;
const int ready = 1e9 + 7;
const int base = 256;
const int inf = 0x0d000721ll << 2 | 0xAC66666;
 
namespace ttq012 {
int read() {
    int o = 1, x = 0;
    char ch;
    while (!isdigit(ch = getchar())) {
        if (ch == '-') {
            o = -o;
        }
    }
    x = ch & 15;
    while (isdigit(ch = getchar())) {
        x = (x << 3) + (x << 1) + (ch & 15);
    }
    return x * o;
}
 
int exgcd(int a, int b, int &x, int &y) {
    if (!b) {
        x = 1, y = 0;
        return a;
    }
    int g, xp, yp;
    g = exgcd(b, a % b, xp, yp);
    x = yp, y = xp - yp * (a / b);
    return g;
}
 
int ksm(int a, int b, int c) {
    a %= c;
    int ans = 1;
    while (b) {
        if (b & 1) {
            ans = ans * a % c;
        }
        a = a * a % c;
        b >>= 1;
    }
    return ans;
}
 
int Inv(int a, int ready) {
    return ksm(a, ready - 2, ready);
}
 
// calc a^(b^c)
int calcpow(int a, int b, int c) {
    int k = ksm(b, c, ready - 1);
    if (k >= ready - 1) {
        k = k % (ready - 1) + (ready - 1);
    }
    return ksm(a, k, ready);
}
 
int gcd(int a, int b) {
    return b ? gcd(b, a % b) : a;
}
 
int lcm(int a, int b) {
    return a / gcd(a, b) * b;
}
 
void swap(int &a, int &b) {
    a ^= b ^= a ^= b;
} } // using namespace ttq012;
 
namespace ttq012 {
 
struct myhash{
    static uint64_t rand(uint64_t x){
        x^=x<<13;
        x^=x>>7;
        return x^=x<<17;
    }
    size_t operator()(size_t x)const{
        static const uint64_t f_rand=chrono::steady_clock::now().time_since_epoch().count();
        return rand(x+f_rand);
    }
};
int a[N], vis[N], z[N];
void add(int x, int v) {
    for (; x < N; x += (x &- x)) {
        z[x] += v;
    }
}
int query(int x) {
    int s = 0;
    for (; x; x -= (x &- x)) {
        s += z[x];
    }
    return s;
}
int f[N], b[N];
void run() {
    int n = read(), k = read(), all = 0;
    for (int i = 1; i <= n; ++i) {
        a[i] = read();
        b[i] = a[i];
    }
    sort(b + 1, b + n + 1);
    for (int i = 1; i <= k; ++i) {
        vector<int> v, w;
        for (int j = i; j <= n; j += k) {
            v.eb(a[j]);
            w.eb(b[j]);
        }
        vector<int> ww = v;
        sort(ww.begin(), ww.end());
        sort(w.begin(), w.end());
        if (w != ww) {
            cout << "-1\n";
            return;
        }
    }
    set<pair<int, int>> se;
    map<int, int> mp, box;
    for (int i = 1; i <= n; ++i) {
        ++box[a[i]];
    }
    for (int i = 1; i <= n; ++i) {
        se.insert({a[i], box[a[i]]});
    }
    int idx = 1;
    for (auto &x : se) {
        mp[x.first] = idx;
        ++idx;
    }
    for (int i = 1; i <= n; ++i) {
        a[i] = mp[a[i]];
    }
    for (int i = 1; i <= k; ++i) {
        vector<int> v, w;
        for (int j = i; j <= n; j += k) {
            v.eb(a[j]);
        }
        for (auto &j : v) {
            all += query(N - 1) - query(j);
            add(j, 1);
        }
        for (auto &j : v) {
            add(j, -1);
        }
    }
    cout << all << '\n';
} }
 
 
signed main() {
    ttq012::run();
    return 0;
}
 
/*
 
 
 
*/