Codeforces Round #237 (Div. 2)__Restore Graph

const int MAXN = 1000100;

vector<int> vt[MAXN];
typedef pair<int, int> pii;
vector<pii> ans;

int main()
{
//    freopen("in.txt", "r", stdin);
    int n, k, t;
    while (~RII(n, k))
    {
        int Max = -INF;
        REP(i, MAXN) vt[i].clear();
        ans.clear();
        FE(i, 1, n)
        {
            RI(t);
            Max = max(Max, t);
            vt[t].push_back(i);
        }
        if (vt[0].size() != 1)
            puts("-1");
        else
        {
            if (Max >= 1 && vt[1].size() > k)
            {
                puts("-1");
                goto end;
            }
            FF(i, 0, Max)
            {
                int cur = vt[i].size();
                int next = vt[i + 1].size();
                if (i != 0 && next > (LL)(k - 1) * cur)
                {
                    puts("-1");
                    goto end;
                }
                REP(j, next)
                    ans.push_back(MP(vt[i][j % cur], vt[i + 1][j]));
            }
            cout << ans.size() << endl;
            REP(i, ans.size())
                cout << ans[i].first << ‘ ‘ << ans[i].second << endl;
        }
        end:;
    }
    return 0;
}