#include <bits/stdc++.h>
using namespace std;
int main() {
  int k, n, a[100001];
  cin >> k >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  if ((k - 1) % (n - 1) == 0)
    cout << (k - 1) / (n - 1) << endl;
  else
    cout << (k - 1) / (n - 1) + 1 << endl;
  return 0;
}
