#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const long long maxn = 1e6 + 7;
inline long long read() {
  long long res = 0, tmp = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') tmp = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    res = (res << 1) + (res << 3) + (ch ^ 48), ch = getchar();
  return res * tmp;
}
inline void write(long long x) {
  if (x < 0) putchar('-'), x = -x;
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
void fre() {
  freopen(".in", "r", stdin);
  freopen(".out", "w", stdout);
}
long long max(long long a, long long b) { return a > b ? a : b; }
long long min(long long a, long long b) { return a < b ? a : b; }
void hour() {
  cerr << "运行时间：" << 1.0 * clock() / CLOCKS_PER_SEC << "s" << endl;
}
long long n, m, ans, f[maxn], t[maxn];
int main() {
  n = read();
  m = read();
  for (register long long i = 1; i <= n - 1; i++)
    for (register long long j = i + 1; j <= n; j++) {
      long long a = read();
      ans += (a >= 0 ? a : 0);
      f[i] += (a >= 0 ? a : 0);
      t[i] += (a >= 0 ? 1 : 0);
      f[j] += (a >= 0 ? a : 0);
      t[j] += (a >= 0 ? 1 : 0);
    }
  if (m != 2)
    ans *= 2;
  else {
    ans = 0;
    long long l = n;
    n = 0;
    for (register long long i = 1; i <= l; i++) {
      t[i] -= 1;
      n += t[i] * (t[i] + 1) / 2;
      ans += f[i] * t[i];
    }
  }
  write(ans / n);
  return 0;
}
