#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, -1, 0, 1};
int dy[] = {1, 0, -1, 0};
int c[2010][2010], edge[2010];
long double s[2010], C[2010][2010];
int main(int argc, char *argv[]) {
  int n, k;
  scanf("%d%d", &n, &k);
  for (int i = (1); i <= (n); ++i) {
    C[i][0] = 1;
    for (int j = (1); j <= (i); ++j) C[i][j] = C[i][j - 1] * (i - j + 1) / j;
  }
  for (int i = (1); i <= (n - 1); ++i)
    for (int j = (i + 1); j <= (n); ++j) {
      scanf("%d", &c[i][j]);
      c[j][i] = c[i][j];
    }
  long double m = 0;
  for (int i = (1); i <= (n); ++i) {
    edge[i] = 0;
    for (int j = (1); j <= (n); ++j)
      if (c[i][j] >= 0 && i != j) edge[i]++;
    s[i] = 0;
    if (edge[i] < k) continue;
    s[i] = C[edge[i]][k];
    m += s[i];
  }
  long double ans = 0;
  for (int i = (1); i <= (n); ++i) {
    if (edge[i] <= 0) continue;
    long double p = (s[i] * k) / (m * edge[i]);
    long double sum = 0;
    for (int j = (1); j <= (n); ++j)
      if (c[i][j] >= 0 && i != j) sum += (long double)c[i][j];
    ans += sum * p;
  }
  ans = ans + 1e-5;
  printf("%I64d\n", (long long)ans);
  return 0;
}
