#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
std::istream& operator>>(std::istream& i, pair<T, U>& p) {
  i >> p.first >> p.second;
  return i;
}
template <typename T>
std::istream& operator>>(std::istream& i, vector<T>& t) {
  for (auto& v : t) {
    i >> v;
  }
  return i;
}
template <typename T, typename U>
std::ostream& operator<<(std::ostream& o, const pair<T, U>& p) {
  o << p.first << ' ' << p.second;
  return o;
}
template <typename T>
std::ostream& operator<<(std::ostream& o, const vector<T>& t) {
  if (t.empty()) o << '\n';
  for (size_t i = 0; i < t.size(); ++i) {
    o << t[i] << " \n"[i == t.size() - 1];
  }
  return o;
}
template <typename T>
using minheap = priority_queue<T, vector<T>, greater<T>>;
template <typename T>
using maxheap = priority_queue<T, vector<T>, less<T>>;
template <typename T>
bool in(T a, T b, T c) {
  return a <= b && b < c;
}
unsigned int logceil(int first) {
  return 8 * sizeof(int) - __builtin_clz(first);
}
namespace std {
template <typename T, typename U>
struct hash<pair<T, U>> {
  hash<T> t;
  hash<U> u;
  size_t operator()(const pair<T, U>& p) const {
    return t(p.first) ^ (u(p.second) << 7);
  }
};
}  // namespace std
template <typename T, typename F>
T bsh(T l, T h, const F& f) {
  T r = -1, m;
  while (l <= h) {
    m = (l + h) / 2;
    if (f(m)) {
      l = m + 1;
      r = m;
    } else {
      h = m - 1;
    }
  }
  return r;
}
template <typename F>
double bshd(double l, double h, const F& f, double p = 1e-9) {
  unsigned int r = 3 + (unsigned int)log2((h - l) / p);
  while (r--) {
    double m = (l + h) / 2;
    if (f(m)) {
      l = m;
    } else {
      h = m;
    }
  }
  return (l + h) / 2;
}
template <typename T, typename F>
T bsl(T l, T h, const F& f) {
  T r = -1, m;
  while (l <= h) {
    m = (l + h) / 2;
    if (f(m)) {
      h = m - 1;
      r = m;
    } else {
      l = m + 1;
    }
  }
  return r;
}
template <typename F>
double bsld(double l, double h, const F& f, double p = 1e-9) {
  unsigned int r = 3 + (unsigned int)log2((h - l) / p);
  while (r--) {
    double m = (l + h) / 2;
    if (f(m)) {
      h = m;
    } else {
      l = m;
    }
  }
  return (l + h) / 2;
}
template <typename T>
T gcd(T a, T b) {
  if (a < b) swap(a, b);
  return b ? gcd(b, a % b) : a;
}
template <typename T>
class vector2 : public vector<vector<T>> {
 public:
  vector2() {}
  vector2(size_t a, size_t b, T t = T())
      : vector<vector<T>>(a, vector<T>(b, t)) {}
};
template <typename T>
class vector3 : public vector<vector2<T>> {
 public:
  vector3() {}
  vector3(size_t a, size_t b, size_t c, T t = T())
      : vector<vector2<T>>(a, vector2<T>(b, c, t)) {}
};
template <typename T>
class vector4 : public vector<vector3<T>> {
 public:
  vector4() {}
  vector4(size_t a, size_t b, size_t c, size_t d, T t = T())
      : vector<vector3<T>>(a, vector3<T>(b, c, d, t)) {}
};
template <typename T>
class vector5 : public vector<vector4<T>> {
 public:
  vector5() {}
  vector5(size_t a, size_t b, size_t c, size_t d, size_t e, T t = T())
      : vector<vector4<T>>(a, vector4<T>(b, c, d, e, t)) {}
};
class cielandgondolas {
 public:
  vector<vector<int>> P;
  inline int get(int i, int j) { return P[j][j] + P[i][i] - P[j][i] - P[i][j]; }
  void dnc(const vector<int>& src, vector<int>& tar, int left, int right,
           int lo, int hi) {
    if (left >= right) return;
    int center = (left + right) / 2;
    std::pair<int, int> best = {src[lo] + get(lo, center), lo};
    for (int i = lo + 1; i < hi; ++i)
      best = min(best, {src[i] + get(i, center), i});
    tar[center] = best.first;
    dnc(src, tar, left, center, lo, best.second + 1);
    dnc(src, tar, center + 1, right, best.second, hi);
  }
  void solve(istream& cin, ostream& cout) {
    int N, K;
    cin >> N >> K;
    P = vector2<int>(N + 1, N + 1, 0);
    for (int i = 0; i < N; ++i) {
      for (int j = 0; j < N; ++j) {
        char c;
        cin >> c;
        P[i + 1][j + 1] = c - '0' + P[i + 1][j] + P[i][j + 1] - P[i][j];
      }
    }
    vector<int> D(N + 1, 1e9);
    vector<pair<std::pair<int, int>, std::pair<int, int>>> Q(N + 1);
    Q[0] = {{0, N + 1}, {0, N + 1}};
    D[0] = 0;
    for (int k = 0; k < K; ++k) {
      vector<int> E(N + 1, 0);
      int j = 1;
      for (int i = 0; i <= N; ++i) {
        auto q = Q[i];
        int center = (q.first.first + q.first.second) / 2;
        std::pair<int, int> best = {
            D[q.second.first] + get(q.second.first, center), q.second.first};
        for (int i = q.second.first + 1; i < q.second.second; ++i)
          best = min(best, {D[i] + get(i, center), i});
        E[center] = best.first;
        if (q.first.first < center)
          Q[j++] = {{q.first.first, center}, {q.second.first, best.second + 1}};
        if (center + 1 < q.first.second)
          Q[j++] = {{center + 1, q.first.second},
                    {best.second, q.second.second}};
      }
      swap(E, D);
    }
    cout << D[N] / 2;
  }
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  cielandgondolas solver;
  std::istream& in(std::cin);
  std::ostream& out(std::cout);
  solver.solve(in, out);
  return 0;
}
