#include <bits/stdc++.h>
using namespace std;
const int P = 1e9 + 7, P2 = 998244353, INF = 0x3f3f3f3f;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long qpow(long long a, long long n) {
  long long r = 1 % P;
  for (a %= P; n; a = a * a % P, n >>= 1)
    if (n & 1) r = r * a % P;
  return r;
}
long long inv(long long first) {
  return first <= 1 ? 1 : inv(P % first) * (P - P / first) % P;
}
inline int rd() {
  int first = 0;
  char p = getchar();
  while (p < '0' || p > '9') p = getchar();
  while (p >= '0' && p <= '9') first = first * 10 + p - '0', p = getchar();
  return first;
}
const int N = 1e5 + 10;
int n, m, pos, a[N];
int chk(long long first) {
  int cnt = 1;
  long long now = first - pos;
  for (int i = pos; i >= 1; --i)
    if (a[i]) {
      if (now >= a[i])
        now -= a[i];
      else {
        int t = a[i] - now;
        while (t >= first - i) ++cnt, t -= first - i;
        if (t)
          ++cnt, now = first - t - i;
        else
          now = 0;
      }
    }
  return cnt <= m;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; ++i) {
    scanf("%d", a + i);
    if (a[i]) pos = i;
  }
  if (!pos) return puts("0"), 0;
  long long l = pos + 1, r = 1e15, ans = -1;
  while (l <= r) {
    if (chk(((l + r) >> 1)))
      ans = ((l + r) >> 1), r = ((l + r) >> 1) - 1;
    else
      l = ((l + r) >> 1) + 1;
  }
  printf("%lld\n", ans);
}
