#include <bits/stdc++.h>
/*
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>
namespace mp = boost::multiprecision;
using bint = mp::cpp_int;
*/
#include <atcoder/all>
#define rep(i,n) for (int i = 0; i < int(n); ++i)
#define repp(i,n,m) for (int i = m; i < int(n); ++i)
#define repb(i,n) for (int i = int(n)-1; i >= 0; --i)
#define endl "\n"
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
using P = pair<int, int>;
using PL = pair<long long, long long>;
using pdd = pair<long double, long double>;
using pil = pair<int,ll>;
using pli = pair<ll,int>;
using ppi = pair<P,int>;
using pip = pair<int,P>;
const int INF = 1001001007;
const long long mod1 = 1000000007LL;
const long long mod2 = 998244353LL;
const ll inf = 2e18;
const ld pi = 3.14159265358979323;
const ld eps = 1e-7;
const char _ = ' ';
template<typename T>void o(T a);
template<class T>istream &operator>>(istream &is,vector<T> &v){for(auto &e:v)is>>e;return is;}
template<class T>ostream &operator<<(ostream &os,const vector<T> &v){if(v.size()!=0){rep(i,v.size())os<<v[i]<<(i+1==v.size()?"":" ");}return os;}
template<class T>istream &operator>>(istream &is,vector<vector<T>> &v){for(auto &e:v)is>>e;return is;}
template<class T>ostream &operator<<(ostream &os,const vector<vector<T>> &v){if(v.size()!=0){for(auto &e:v)o(e);}return os;}
template<typename T>bool range(T a,T b,T x){return (a<=x&&x<b);}
template<typename T>bool rrange(T a,T b,T c,T d,T x,T y){return (range(a,c,x)&&range(b,d,y));}
template<typename T>void rev(vector<T> &v){reverse(v.begin(),v.end());}
void revs(string &s) {reverse(s.begin(),s.end());}
template<typename T>void sor(vector<T> &v, int f=0){sort(v.begin(),v.end());if(f!=0) rev(v);}
template<typename T>bool chmin(T &a,const T &b){if(a>b){a=b;return true;}return false;}
template<typename T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<typename T>void eru(vector<T> &v){sor(v);v.erase(unique(v.begin(),v.end()),v.end());}
template<typename T>T cel(T a,T b){if(a%b==0)return a/b;return a/b +1;}
void o(){cout<<"!?"<<endl;}
template<typename T>void o(T a){cout<<a<<endl;}

template<typename T>void mout(T a){cout<<a.val()<<endl;}
void print(){ putchar(' '); }
void print(bool a){ printf("%d", a); }
void print(int a){ printf("%d", a); }
void print(long a){ printf("%ld", a); }
void print(long long a){ printf("%lld", a); }
void print(char a){ printf("%c", a); }
void print(char a[]){ printf("%s", a); }
void print(const char a[]){ printf("%s", a); }
void print(long double a){ printf("%.15Lf", a); }
void print(const string& a){ for(auto&& i : a) print(i); }
template<class T> void print(const T& a){ cout << a; }
int out(){ putchar('\n'); return 0; }
template<class T> int out(const T& t){ print(t); putchar('\n'); return 0; }
template<class Head, class... Tail> int out(const Head& head, const Tail&... tail){ print(head); putchar(' '); out(tail...); return 0; }
void yes(){cout << "Yes" << endl;}
void no (){cout << "No" << endl;}
void yn (bool t){if(t)yes();else no();}
template<typename T>void dame(bool t, T s){if(!t){cout << s << endl;exit(0);}}
void fast_io(){cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(20);}
template<typename T,typename U>void o2(pair<T,U> a){out(a.first,a.second);}
vector<int> dx = {0,1,0,-1,1,1,-1,-1};
vector<int> dy = {1,0,-1,0,1,-1,-1,1};
const string ALP = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const string alp = "abcdefghijklmnopqrstuvwxyz";
const string NUM = "0123456789";

ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}
ll mpow(ll x,ll n,ll m){if(n==0)return 1LL;x%=m;ll a=mpow(x,n/2,m);a=a*a%m;return (n&1)?a*x%m:a;}

template<typename T> ll tentou(vector<T> ar){
    int n = ar.size();
    set<T> st;
    rep(i,n) st.insert(ar[i]);
    map<T,int> mp;
    int ind = 0;
    for (T x : st){
        mp[x] = ind;
        ind++;
    }
    fenwick_tree<ll> fw(ind);
    ll ans = 0;
    rep(i,n){
        int a = mp[ar[i]];
        ans += i - fw.sum(0,a+1);
        fw.add(a,1);
    }
    return ans;
}

struct edge{
    int from, to;
    long long cost;
    edge(int _from = -1, int _to = -1, long long _cost = 1LL) : from(_from), to(_to), cost(_cost) {}
};

struct vertex{
    vector<edge> adj;
};

struct Graph{
    int n;
    vector<vertex> vs;
    void add_edge(int from, int to, long long cost = 1LL){
        assert(0 <= from && from < n);
        assert(0 <= to && to < n);
        vs[from].adj.emplace_back(edge(from,to,cost));
    }
    void add_dual_edge(int from, int to, long long cost = 1LL){
        assert(0 <= from && from < n);
        assert(0 <= to && to < n);
        vs[from].adj.emplace_back(edge(from,to,cost));
        vs[to].adj.emplace_back(edge(to,from,cost));
    }
    Graph(int _n) : n(_n) , vs(n) {}
    vector<long long> dijkstra(int s){
        using pli = pair<long long, int>;
        priority_queue<pli, vector<pli>, greater<pli>> pque;
        vector<long long> dist(n,inf);
        dist[s] = 0LL;
        pque.push(pli(0,s));
        while (!pque.empty()){
            pli p = pque.top(); pque.pop();
            if (dist[p.second] < p.first) continue;
            for (edge x : vs[p.second].adj){
                if (dist[x.to] > p.first + x.cost){
                    dist[x.to] = p.first + x.cost;
                    pque.push(pli(dist[x.to],x.to));
                }
            }
        }
        return dist;
    }
    vector<long long> bfs01(int s){
        deque<int> que;
        vector<long long> dist(n,inf);
        dist[s] = 0LL;
        que.push_front(s);
        while (!que.empty()){
            int p = que.front(); que.pop_front();
            for (edge x : vs[p].adj){
                if (dist[x.to] > dist[p] + x.cost){
                    dist[x.to] = dist[p] + x.cost;
                    if (x.cost == 0LL) que.push_front(x.to);
                    else que.push_back(x.to);
                }
            }
        }
        return dist;
    }
    vector<int> dfs(int s){
        vector<int> ans;
        vector<int> vis(n,0);
        _dfs(s,ans,vis);
        return ans;
    }
    private:
    void _dfs(int s, vector<int> &ans, vector<int> &vis){
        vis[s]++;
        for (edge x : vs[s].adj){
            if (vis[x.to] == 0){
                _dfs(x.to,ans,vis);
            }
        }
        ans.emplace_back(s);
    }
};

struct Tree{
    Tree(int _n, int _root = 0) : n(_n), root(_root) {
        assert(0 <= root && root < n);
        initialize();
    }
    void add_edge(int from, int to, long long cost = 1LL){
        assert(0 <= from && from < n);
        assert(0 <= to && to < n);
        vs[from].adj.emplace_back(edge(from,to,cost));
    }
    void add_dual_edge(int from, int to, long long cost = 1LL){
        assert(0 <= from && from < n);
        assert(0 <= to && to < n);
        vs[from].adj.emplace_back(edge(from,to,cost));
        vs[to].adj.emplace_back(edge(to,from,cost));
    }
    int size(){return n;}
    int parent(int v){
        assert(0 <= v && v < n);
        if (is_done_par_rdist_init == false) par_rdist_init();
        return par[v];
    }
    int depth(int v){
        assert(0 <= v && v < n);
        if (dep[v] != -1) return dep[v];
        if (v == root) return dep[v] = 0;
        return dep[v] = depth(parent(v)) + 1;
    }
    int subtree_size(int v){
        assert(0 <= v && v < n);
        if (sub[v] != 0) return sub[v];
        sub[v] = 1;
        for (edge x : vs[v].adj){
            if (x.to != parent(v)) sub[v] += subtree_size(x.to);
        }
        return sub[v];
    }
    int lca(int u, int v){
        assert(0 <= u && u < n);
        assert(0 <= v && v < n);
        if (is_done_lca_init == false) lca_init();
        if (depth(u) > depth(v)) swap(u,v);
        for (int i = 0; i < 30; i++) if ((depth(v) - depth(u)) >> i & 1) v = par2[i][v];
        if (u == v) return u;
        for (int k = 29; k >= 0; k--){
            if (par2[k][u] != par2[k][v]) {
                u = par2[k][u];
                v = par2[k][v];
            }
        }
        return par2[0][u];
    }
    long long dist(int u, int v){
        assert(0 <= u && u < n);
        assert(0 <= v && v < n);
        if (is_done_par_rdist_init == false) par_rdist_init();
        return rdist[u] + rdist[v] - rdist[lca(u,v)] * 2LL;
    }
    vector<int> path(int f, int t){
        assert(0 <= f && f < n);
        assert(0 <= t && t < n);
        int v = lca(f,t);
        vector<int> fp = {f};
        vector<int> tp = {t};
        int fn = f, tn = t;
        while (fn != v){
            fn = parent(fn);
            fp.emplace_back(fn);
        }
        while (tn != v){
            tn = parent(tn);
            tp.emplace_back(tn);
        }
        for (int i = int(tp.size()) - 2; i >= 0; i--){
            fp.emplace_back(tp[i]);
        }
        return fp;
    }
    vector<long long> alldists(int v){
        assert(0 <= v && v < n);
        if (is_done_par_rdist_init == false) par_rdist_init();
        if (v == 0) return rdist;
        vector<long long> dists(n,1e18);
        vector<int> vis(n,0);
        dists[v] = 0LL;
        queue<int> que;
        que.push(v);
        while (!que.empty()){
            int p = que.front(); que.pop();
            vis[p]++;
            for (edge x : vs[p].adj){
                if (vis[x.to] == 0){
                    dists[x.to] = dists[p] + x.cost;
                    que.push(x.to);
                }
            }
        }
        return dists;
    }
    vector<int> dfs(int v){
        assert(0 <= v && v < n);
        vector<int> ans;
        vector<int> vis(n,0);
        _dfs(v,vis,ans);
        return ans;
    }
    vector<vertex> vs;
    private:
    int n;
    int root;
    bool is_done_lca_init;
    bool is_done_par_rdist_init;
    vector<int> par;
    vector<int> dep;
    vector<int> sub;
    vector<long long> rdist;
    vector<vector<int>> par2;
    void initialize(){
        is_done_lca_init = false;
        is_done_par_rdist_init = false;
        vs.resize(n);
        dep.resize(n,-1);
        sub.resize(n,0);
    }
    void lca_init(){
        par2.resize(30,vector<int>(n,-1));
        for (int i = 0; i < n; i++) par2[0][i] = parent(i);
        for (int i = 0; i < 29 ; i++) {
            for (int j = 0; j < n; j++) {
                if (par2[i][j] < 0) par2[i+1][j] = -1;
                else par2[i+1][j] = par2[i][par2[i][j]];
            }
        }
        is_done_lca_init = true;
    }
    void par_rdist_init(){
        par.resize(n,-2);
        rdist.resize(n,-1);
        par[root] = -1;
        rdist[root] = 0;
        queue<int> que;
        que.push(root);
        while (!que.empty()){
            int p = que.front(); que.pop();
            for (edge x : vs[p].adj){
                if (par[x.to] == -2){
                    par[x.to] = p;
                    rdist[x.to] = rdist[p] + x.cost;
                    que.push(x.to);
                }
            }
        }
        is_done_par_rdist_init = true;
    }
    void _dfs(int v, vector<int> &vis, vector<int> &ans){
        vis[v]++;
        for (edge x : vs[v].adj){
            if (vis[x.to] == 0) _dfs(x.to,vis,ans);
        }
        ans.emplace_back(v);
    }
};

template<typename T> struct doubling{
    vector<T> vec;
    doubling (vector<T> _vec) : vec(_vec) {}
    map<T,ll> mp;
    ll length_of_loop = -1;
    ll top_of_loop = -1;
    void init(){
        ll ind = 0;
        for (T x : vec){
            if (mp.find(x) == mp.end()) mp[x] = ind, ind++;
            else {
                length_of_loop = ind - mp[x];
                top_of_loop = mp[x];
                break;
            }
        }
    }
    ll len(){
        if (length_of_loop == -1) init();
        return length_of_loop;
    }
    ll top(){
        if (top_of_loop == -1) init();
        return top_of_loop;
    }
    T get(ll n){
        assert(0 <= n);
        if (n < top()) return vec[n];
        ll d = n - top();
        return vec[top() + (d % len())];
    }
};

struct Mo{
    vector<int> left, right, order, v;
    int width, nl, nr, ptr;
    Mo (int n = 0) : width(sqrt(n)), nl(0), nr(0), ptr(0), left(0), right(0), v(n,-1) {}
    void insert(int l, int r){ // [l,r)
        left.emplace_back(l);
        right.emplace_back(r);
    }
    void build(){ // sort all query
        order.resize(left.size());
        iota(order.begin(),order.end(),0);
        sort(order.begin(),order.end(),[&](int a, int b){
            if(left[a] / width != left[b] / width) return left[a] < left[b];
            return right[a] < right[b];
        });
    }
    int process(){ // do 1 query
        if (ptr == int(order.size())) return -1;
        const int id = order[ptr];
        while (nl > left[id]) distribute(--nl);
        while (nl < left[id]) distribute(nl++);
        while (nr > right[id]) distribute(--nr);
        while (nr < right[id]) distribute(nr++);
        return order[ptr++];
    }
    inline void distribute(int idx){ // x x x (nl) o o o (nr) x x ...
        v[idx] *= -1;
        if (v[idx] == 1) add(idx);
        else del(idx);
    }
    void add(int idx);
    void del(int idx);
};

template<class S, S op(S l, S r), S e()> struct Rui{
    /*
    - op(e,x) = op(x,e) = x
    - op(l,opinv(l,lr)) = lr
    */
    int n;
    vector<S> vec, lrui, rrui;
    Rui (vector<S> _vec) : n(int(_vec.size())), vec(_vec) {
        lrui.resize(n+1), rrui.resize(n+1);
        lrui[0] = rrui[n] = e();
        for (int i = 0; i < n; i++) lrui[i+1] = op(lrui[i],vec[i]);
        for (int i = n; i > 0; i--) rrui[i-1] = op(rrui[i],vec[i-1]);
    }
    template<S opinv(S l, S lr)> S prod(int l, int r = -1){ // [l,r)
        if (r == -1) return vec[l];
        return opinv(lrui[l],lrui[r]);
    }
    S nprod(int l, int r = -1){ // [0,l) * [r,n)
        if (r == -1) r = l+1;
        return op(lrui[l],rrui[r]);
    }
};

struct MINT998244353{
    using mint = modint998244353;
    vector<mint> kaijo, kainv;
    int N;
    MINT998244353 (int lim = 200000) : N(lim), kaijo(lim+1,1), kainv(lim+1,1) {
        rep(i,N) kaijo[i+1] = kaijo[i] * (i+1);
        rep(i,N) kainv[i+1] = kainv[i] / (i+1);
    }
    mint factrial(int x){return kaijo[x];}
    mint inv_factorial(int x){return kainv[x];}
    mint ncr(int n, int r){return kaijo[n] * kainv[r] * kainv[n-r];}
    mint npr(int n, int r){return kaijo[n] * kainv[n-r];}
    template<typename T> vector<mint> beki(T r, int n = -1){
        if (n == -1) n = N;
        vector<mint> res(n+1,1);
        rep(i,n) res[i+1] = res[i] * mint(r);
        return res;
    }
    vector<mint> vec_inv(vector<mint> &a){
        vector<mint> res(a.size());
        rep(i,int(a.size())) res[i] = a[i].inv();
        return res;
    }
};

struct MINT1000000007{
    using mint = modint1000000007;
    vector<mint> kaijo, kainv;
    int N;
    MINT1000000007 (int lim = 200000) : N(lim), kaijo(lim+1,1), kainv(lim+1,1) {
        rep(i,N) kaijo[i+1] = kaijo[i] * (i+1);
        rep(i,N) kainv[i+1] = kainv[i] / (i+1);
    }
    mint factrial(int x){return kaijo[x];}
    mint inv_factorial(int x){return kainv[x];}
    mint ncr(int n, int r){
        if (n < r) return mint(0);
        return kaijo[n] * kainv[r] * kainv[n-r];
    }
    template<typename T> vector<mint> beki(T r, int n = -1){
        if (n == -1) n = N;
        vector<mint> res(n+1,1);
        rep(i,n) res[i+1] = res[i] * mint(r);
        return res;
    }
    vector<mint> vec_inv(vector<mint> &a){
        vector<mint> res(a.size());
        rep(i,int(a.size())) res[i] = a[i].inv();
        return res;
    }
};

/* 
ll op(ll a, ll b){
    return a + b;
}
ll e(){return 0;}
ll opinv(int a, int ab){return ab - a;} */


template<typename T> void vmint(vector<T> &v){
    int n = v.size();
    if (n == 0) {
        cout << endl;
        return ;
    }
    rep(i,n) cout << v[i].val() << (i < n-1 ? " " : "\n");
}

template<typename T> void vvmint(vector<vector<T>> &v){
    int n = v.size();
    if (n == 0) {
        cout << endl;
        return ;
    }
    rep(i,n) vmint(v[i]);
}


#include<random>
random_device rd;
mt19937_64 mt(rd());

void shf(vector<int> &a){
    int n = a.size();
    rep(i,n){
        int j = i + (mt() % (n - i));
        swap(a[i],a[j]);
    }
}

struct stackmax{
    stack<ll> st, ma;
    void push(ll x){
        st.push(x);
        if (!ma.empty()) chmax(x,ma.top());
        ma.push(x);
    }
    void pop(){ st.pop(); ma.pop();}
    ll top(){return st.top();}
    ll getmax(){
        if (ma.empty()) return -inf;
        return ma.top();
    }
    bool empty(){return st.empty();}
};

struct queuemax{
    stackmax le, ri; // << (le] [ri) <<
    void push(ll x){ri.push(x);}
    void pop(){
        if (le.empty()){
            while (!ri.empty()){
                le.push(ri.top()); ri.pop();
            }
        }
        le.pop();
    }
    ll getmax(){return max(le.getmax(),ri.getmax());}
};


void solve(){
    int n, k; cin >> n >> k;
    if (2 * k < n){
        o(-1);
        return ;
    }
    vector<P> ans;
    rep(i,k) ans.emplace_back(P(i+1,i+1));
    rep(i,n-k){
        ans.emplace_back(P(k+i+1,i+1));
        ans.emplace_back(P(i+1,k+i+1));
    }
    o(ans.size());
    for (auto p : ans) out(p.first,p.second);
}


int main(){
    fast_io();
    int t = 1; //cin >> t;
    while (t--) solve();
}