#include <cstdio>
#include <cstring>
#include <algorithm>
#define maxN 1002
using namespace std;
int n, i, j, k, dp[maxN][maxN], sol, sum, sd;
int sl[maxN][maxN], sc[maxN][maxN], si[2 * maxN][maxN];
void read()
{
    int x;
    freopen("ferma2.in", "r", stdin);
    scanf("%d %d", &n, &k);
    for (i = 1; i <= n; ++ i)
    {
        for (j = 1; j <= i; ++ j)
        {
            scanf("%d", &x);
            sum += x;
            sl[n - i + 1][j] = sl[n - i + 1][j - 1] + x;
            sc[n - i + 1][j] = sc[n - i + 2][j] + x;
            si[n - i + 1 + j][j] = si[n - i + 1 + j][j - 1] + x;
            if (i == n && j >= n - k + 1)
                sd += si[n - i + 1 + j][j];
        }
    }
}
void solve()
{
    int i, j;
    dp[1][1] = sum - sd;
    sol = dp[1][1];
    -- k;
    for (i = 1; i + k - 1 <= n; ++ i)
        for (j = 1; j + k - 1 <= n - i + 1; ++ j)
            if (i + j > 2)
            {
                dp[i][j] = si[i + j + k - 1][j + k - 1] - si[i + j + k - 1][j - 1];
                if (i > 1)
                    dp[i][j] += dp[i - 1][j] - sl[i - 1][j + k - 1] + sl[i - 1][j - 1];
                else
                    dp[i][j] += dp[i][j - 1] - sc[i][j - 1] + sc[i + k][j - 1];

                if (dp[i][j] < sol)
                    sol = dp[i][j];
            }
}
void write()
{
    freopen("ferma2.out", "w", stdout);
    printf("%d\n", sum - sol);
}
int main()
{
    read();
    solve();
    write();
    return 0;
}