#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <functional>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <deque>
#include <queue>

#define ft first
#define sc second
#define mp make_pair
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define sz(a) (int)(a).size()
#define forn(i, n) for (int i = 0; i < (n); i++)
#define fore(i, f, t) for (int i = (f); i < (t); i++)
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>

using namespace std;

const int N = 2010;
int a[N][N], cnt[N];
ll sum[N];


int main(){
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    int n, k;
    scanf("%d%d", &n, &k);
    forn(i, n - 1)
        fore(j, i + 1, n){
            scanf("%d", &a[i][j]);
            a[j][i] = a[i][j];
            if (a[i][j] != -1){
                cnt[i]++;
                cnt[j]++;
            }
        }

    forn(i, n)
        forn(j, n)
            sum[i] += max(a[i][j], 0);

    ll ans = 0, m = 0;
    forn(i, n)
        if (cnt[i] >= k){
            ans += sum[i] * max(1ll, 1ll * (cnt[i] - k) * (cnt[i] - k + 1) / 2);
            m += 1ll * (cnt[i] - k + 1) * (cnt[i] - k + 2) / 2;
        }
    if (m == 0)
        printf("0\n");
    else
        printf("%I64d\n", ans / m);
}