import core.bitop;
import std.algorithm;
import std.ascii;
import std.bigint;
import std.conv;
import std.functional;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.random;
import std.typecons;
import std.container;

alias sread = () => readln.chomp();
alias Point2 = Tuple!(long, "y", long, "x");

T lread(T = long)()
{
    return readln.chomp.to!T();
}

T[] aryread(T = long)()
{
    return readln.split.to!(T[])();
}

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}

void main()
{
    long n, k;
    scan(n, k);
    auto a = aryread();
    long location;
    foreach (i, e; a)
    {
        if (e == 1)
        {
            location = i;
        }
    }
    if (location == 0 || location == n - 1)
    {
        writeln((n - 1) / (k - 1) + !!((n - 1) % (k - 1)));
        return;
    }
    long min_cost = 1000000;
    foreach (i; 0 .. k)
    {
        long cost, right = location, left = n - location - 1;
        if (i >= right)
        {
            cost++;
        }
        else
        {
            right -= i;
            cost += 1 + right / (k - 1) + !!(right % (k - 1));
        }

        if (k - i >= left)
        {
            cost++;
        }
        else
        {
            left -= k - i;
            cost += 1 + left / (k - 1) + !!(left % (k - 1));
        }
        min_cost = min(cost, min_cost);
    }
    writeln(min_cost);
}