#include <bits/stdc++.h>
using namespace std;
inline bool isPowerOfTwo(int x) { return (x != 0 && (x & (x - 1)) == 0); }
double pi = 3.141592653589793238462643;
int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
inline int add(int a, int b, int m = 1000000007) {
  a += b;
  if (a >= m) a -= m;
  return a;
}
inline int mul(int a, int b, int m = 1000000007) {
  return (int)(((long long)a * (long long)b) % m);
}
inline bool ispalin(string &str) {
  int n = str.length();
  for (int i = 0; i < n / 2; i++)
    if (str[i] != str[n - i - 1]) return false;
  return true;
}
long long expo(long long base, long long pow) {
  long long ans = 1;
  while (pow != 0) {
    if (pow & 1 == 1) {
      ans = ans * base;
      ans = ans % 1000000007;
    }
    base *= base;
    base %= 1000000007;
    pow /= 2;
  }
  return ans;
}
void swapp(int *a, int *b) {
  int temp = *a;
  *a = *b;
  *b = temp;
}
long long invt(long long x) { return expo(x, 1000000007 - 2); }
int expFactor(int n, int p) {
  int x = p;
  int exponent = 0;
  while ((n / x) > 0) {
    exponent += n / x;
    x *= p;
  }
  return exponent;
}
inline int countsetbit(int n) {
  unsigned int count = 0;
  while (n) {
    n &= (n - 1);
    count++;
  }
  return count;
}
inline int abs(int x) {
  if (x < 0) return -x;
  return x;
}
bool isPal(string ss) {
  int len = ss.length();
  for (int i = 0; i < len / 2; i++) {
    int comp = len - i - 1;
    if (ss[i] != ss[comp]) return false;
  }
  return true;
}
vector<long long> fact, inv;
void calfal(int maxn) {
  fact.resize(maxn);
  inv.resize(maxn);
  fact[0] = inv[0] = inv[1] = 1;
  for (int i = (1); i <= (maxn - 1); ++i)
    fact[i] = i * fact[i - 1] % 1000000007;
  for (int i = (2); i <= (maxn - 1); ++i)
    inv[i] = (1000000007 - 1000000007 / i) * inv[1000000007 % i] % 1000000007;
  for (int i = (1); i <= (maxn - 1); ++i)
    inv[i] = inv[i] * inv[i - 1] % 1000000007;
}
long long ncr(int n, int r) {
  if (r > n) return 0;
  return (fact[n] * inv[r] % 1000000007) * inv[n - r] % 1000000007;
}
long long _sieve_size;
bitset<10000010> bs;
vector<int> primes;
void sieve(long long upperbound) {
  _sieve_size = upperbound + 1;
  bs.set();
  bs[0] = bs[1] = 0;
  for (long long i = 2; i <= _sieve_size; i++)
    if (bs[i]) {
      for (long long j = i * i; j <= _sieve_size; j += i) bs[j] = 0;
      primes.push_back((int)i);
    }
}
bool isPrime(long long N) {
  if (N <= _sieve_size) return bs[N];
  for (int i = 0; i < (int)primes.size(); i++)
    if (N % primes[i] == 0) return false;
  return true;
}
int fun(long long n, long long m, long long k) {
  long long low = (m) * (m + 1) / 2;
  long long high = k * (k + 1) / 2;
  long long p = k - m;
  high -= p * (p + 1) / 2;
  if ((n >= low && n <= high) || n <= low) return 1;
  return 0;
}
int main() {
  long long n, k;
  cin >> n >> k;
  long long l = 1, h = k - 1;
  if (n == 1) {
    cout << 0 << endl;
    return 0;
  }
  if (n > ((k * (k + 1)) / 2 - k + 1)) {
    cout << "-1" << endl;
    return 0;
  }
  while (l <= h) {
    long long m = (l + h) / 2;
    if (fun(n - 1, m, k - 1)) {
      h = m - 1;
    } else {
      l = m + 1;
    }
  }
  cout << l << endl;
}
