#include <bits/stdc++.h>
#define int long long
void solve() {
int n, k;
std::cin >> n >> k;
std::vector<int> a(n + 1);
for(int i = 1; i <= n; i++) {
std::cin >> a[i];
}
if(*min_element(a.begin() + 1, a.end()) != *max_element(a.begin() + 1, a.end())) {
return;
}
if(n % k == 0) {
std::cout << n / k << "\n";
int i = 1;
while(i <= n) {
for(int j = i; j <= i + k - 1; j++) {
std::cout << j << " ";
}
std::cout << "\n";
i += k;
}
}
else {
std::cout << - 1;
}
}
signed main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
int t = 1;
//std::cin >> t;
while (t--) {
solve();
}
return 0;
}
