#include <bits/stdc++.h>
using namespace std;
#define int ll
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<double> vd;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<pi> vp;
typedef vector<pl> vpl;
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
cerr.tie(nullptr);
int n, k;
cin >> n >> k;
vi a(n);
for (auto& i : a)
cin >> i;
if (n%k) return cout << "-1\n", 0;
cout << n/k << '\n';
for (int i = 1; i <= n; i+=k){
cout << a[0] << ' ';
for (int j = i; j < i+k; ++j)
cout << j << ' ';
cout << '\n';
}
}
