#include <bits/stdc++.h>
using namespace std;
#define _ <<" "<<
#define sz(x) ((int) (x).size())
typedef pair<int, int> pii;
typedef long long ll;
const int maxn = 1e6 + 5;


int n, m;
vector<int> G[maxn];
int id[maxn];

void addedge(int a,int b) {
    G[a].push_back(b);
    G[b].push_back(a);
}

vector<pii> ans;

void dfs(int x,int fa) {
    for (auto v : G[x]) if (v != fa) {
        dfs(v, x);
        ans.push_back({v, x});
    }
}
int sum = 0;

void varify(int x,int fa,int dep) {
    sum += dep;
    for (auto v : G[x]) if (v != fa) {
        varify(v, x, dep+1);
    }
}
void solve(int a,int b) {
    
    if (a < b-1) {
        cout << "impossible\n";
        return;
    }
    int t,n;
    for (int p = 1;p <= 1000000; ++p) if (p*b <= 1000000) {
        ll cur = (ll)(b*p-1)*b*p/2;
        if (a*p >= b*p - 1 && a*p <= cur) {
            n = b*p;
            t = a*p;
            break;
        }
    }
    
    // for (int i = 1;i <= n; ++i) G[i].clear();
    // memset(id,0,sizeof(id));
    ans.clear();
    int tar = t;
    int k = n;
    int maxdep = 0;
    id[0] = n; k--;
    int mx = n;
    // cout << "Tar" _ t _ k << endl;

    while (k <= t-maxdep) {
        addedge(k,mx); mx = k;
        id[++maxdep] = k;
        k--; 
        t -= maxdep;
    }
    //  cout << "Tar" _ t _ k << endl;
    for (int i = 1;i < k; ++i) {
        addedge(n, i);
        t--;
    }
    // cout << "Tar" _ t _ k << endl;
    addedge(id[t-1], k);
    dfs(n,0);
    cout << n _ n-1 << '\n';
    for (auto [a,b] : ans) {
        cout << a _ b << '\n';
    }
    // sum = 0;
    // varify(n,0,0);
    // cout << sum _ tar << endl;
    // assert(sum == tar);
}



int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int a; cin >> a;
    char c; cin >> c;
    int b; cin >> b;

    solve(a,b);

    return 0;
}