#include <bits/stdc++.h>
using namespace std;

#define double long double

using ll = long long;
using VB = vector<bool>;
using VVB = vector<VB>;
using VC = vector<char>;
using VI = vector<int>;
using VVI = vector<VI>;
using VVVI = vector<VVI>;
using VL = vector<ll>;
using VVL = vector<VL>;
using VVVL = vector<VVL>;
using VVVVL = vector<VVVL>;
using VD = vector<double>;
using VVD = vector<VD>;
using VVVD = vector<VVD>;
//using P = pair<int, int>;

#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define FOR(i, a, b) for (ll i = a; i < (ll)(b); i++)
#define ALL(a) (a).begin(),(a).end()

constexpr int INF = 1001001001;
constexpr ll LINF = 1001001001001001001ll;
constexpr int DX[] = {2, 1, -1, -2, -2, -1, 1, 2};
constexpr int DY[] = {1, 2, 2, 1, -1, -2, -2, -1};

template< typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true); }
template< typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true); }

const ll MOD = 998244353;

const int MAX_N = 200010;
int par[MAX_N];
int rnk[MAX_N];
int siz[MAX_N];

void init(int n) {
    REP(i,n) {
        par[i] = i;
        rnk[i] = 0;
        siz[i] = 1;
    }
}

int find(int x) {
    if (par[x] == x) {
        return x;
    } else {
        return par[x] = find(par[x]);
    }
}

void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y) return;
    int s = siz[x] + siz[y];
    if (rnk[x] < rnk[y]) {
        par[x] = y;
    } else {
        par[y] = x;
        if (rnk[x] == rnk[y]) rnk[x]++;
    }
    siz[find(x)] = s;
}

bool same(int x, int y) {
    return find(x) == find(y);
}

int size(int x) {
    return siz[find(x)];
}

ll mod_pow(ll x, ll n, ll mod) {
    ll res = 1;
    while (n > 0) {
        if (n & 1) res = res * x % mod;
        x = x * x % mod;
        n >>= 1;
    }
    return res;
}

typedef pair<ll, int> P0;
struct edge { int to; ll cost; };

const int MAX_V =200000;
//const ll LINF = 1LL<<60;

int V;
vector<edge> G[MAX_V];
ll d[MAX_V];

void dijkstra(ll s) {
    // greater<P>を指定することでfirstが小さい順に取り出せるようにする
    priority_queue<P0, vector<P0>, greater<P0> > que;
    fill(d, d + V, LINF);
    d[s] = 0;
    que.push(P0(0, s));

    while (!que.empty()) {
        P0 p = que.top(); que.pop();
        int v = p.second;
        if (d[v] < p.first) continue;
        REP(i,G[v].size()) {
            edge e = G[v][i];
            if (d[e.to] > d[v] + e.cost) {
                d[e.to] = d[v] + e.cost;
                que.push(P0(d[e.to], e.to));
            }
        }
    }
}

int main() {
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int N, K;
    cin >> N >> K;
    if (N > K * 2) {
        cout << "-1\n";
        return 0;
    }
    cout << N - K + N << "\n";
    REP(i, N - K) {
        cout << i * 2 + 1 << " " << i * 2 + 2 << "\n";
    }
    REP(i, N) cout << i + 1 << " " << i + 1 << "\n";
}