/*
TASK: siuu
*/
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define N 4000005
#define ff first
#define ss second
#define pb push_back
#define sz(s) (int)s.size()
#define rep(c, a, b) for(c = a; c <= b; c++)
ll n, t, m, l, jog;
deque <ll> v, ans;
pair <ll,ll> a[N];
vector <deque<ll>> answer;
int main () {
ios::sync_with_stdio(0);cin.tie(0);
cin >> n >> m;
for(int i = 1;i <= n; i++) {
cin >> a[i].ff;
a[i].ss = i;
}
if(n % m) {
cout << "-1\n";
return 0;
}
cout << (n / m) << '\n';
for(int i = 1; i <= n; i++){
if(i % m == 1) cout << a[i].ff << " ";
cout << i << " ";
if(i % m == 0) cout << '\n';
}
}
