// Problem: C. Football
// Contest: Codeforces - RCC 2014 Warmup (Div. 2)
// URL: https://codeforces.com/problemset/problem/417/C
// Memory Limit: 256 MB
// Time Limit: 1000 ms
// 2022-03-21 14:17:11
// 
// Powered by CP Editor (https://cpeditor.org)

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

#define rep(i,l,r) for(int i=(l);i<=(r);i++)
#define per(i,l,r) for(int i=(l);i>=(r);i--)
#define ll long long
#define mset(s,t) memset(s,t,sizeof(t))
#define mcpy(s,t) memcpy(s,t,sizeof(t))
#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
#define mp make_pair

typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;   
typedef vector<ll> Vll;               
typedef vector<pair<int, int> > vpii;
typedef vector<pair<ll, ll> > vpll;                        

const ll mod = 1e9 + 7;
//const ll mod = 998244353;
const double pi  = acos(-1.0);

inline ll qmi (ll a, ll b) {
    ll ans = 1;
    while (b) {
        if (b & 1) ans = ans * a%mod;
        a = a * a %mod;
        b >>= 1;
    }
    return ans;
}
inline int read () {
    int x = 0, f = 0;
    char ch = getchar();
    while (!isdigit(ch)) f |= (ch=='-'),ch= getchar();
    while (isdigit(ch)) x = x * 10 + ch - '0', ch = getchar();
    return f?-x:x;
}
template<typename T> void print(T x) {
    if (x < 0) putchar('-'), x = -x;
    if (x >= 10) print(x/10);
    putchar(x % 10 + '0');
}
inline ll sub (ll a, ll b) {
    return ((a - b ) %mod + mod) %mod;
}
inline ll add (ll a, ll b) {
    return (a + b) %mod;
}
inline ll inv (ll a) {
    return qmi(a, mod - 2);
}
    vector<pii> ans;

int st[1010][1010];
void solve() {
    int n, k;
    cin >> n >> k;
    if (n == 1) {
        puts("-1");
        return;
    }
    for (int i = 1; i <= n; i ++) {
        for (int j = i + 1, t = k;t;j++, t --){
            if (j > n) {
                j = 1;
            }
            if (st[i][j]) {
                puts("-1");
                return;
            }
            st[i][j] = st[j][i] = 1;
            ans.pb({i, j});
        }
    }
    // cout << ans.size() <<endl;
    printf("%d\n", ans.size());
    for (auto t : ans) {
        printf("%d %d\n", t.fi, t.se);
        // cout << t.fi << ' ' << t.se << endl;
    }
}
int main () {
    // ios::sync_with_stdio(0),cin.tie(0), cout.tie(0);
    int t;
    t =1;
    //cin >> t;
    while (t --) solve();
    return 0;
}

题意n个队，每个队赢k次，每个队只比一次，问合法的表 

题解:构造对于i，我们选择[i + 1, i + k]这范围的队。如果出现两个队之前比过就不对

问题结构:构造匹配问题。通过玩可以规律