import std.conv;
import std.stdio;
import std.string;

void main()
{
	auto nk = readln.split.to!( int[] );
	auto as = readln.split.to!( int[] );
	writeln( solve( nk[ 1 ], as ) );
}

auto solve( in int k, in int[] as )
{
	auto as_r = as.length - k;
	auto k_r = k - 1;
	auto c = 1 + as_r / k_r;
	auto re = ( as_r % k_r == 0 ) ? 0 : 1;

	return c + re;
}

unittest
{
	assert( solve( 3, [ 2, 3, 1, 4 ] ) == 2 );
	assert( solve( 3, [ 1, 2, 3 ] ) == 1 );
	assert( solve( 3, [ 7, 3, 1, 8, 4, 6, 2, 5 ] ) == 4 );

	assert( solve( 3, [ 7, 3, 6, 8, 4, 1, 2, 5 ] ) == 4 );
	assert( solve( 3, [ 7, 3, 6, 1, 4, 8, 2, 5 ] ) == 4 );
	assert( solve( 3, [ 7, 3, 6, 4, 1, 8, 2, 5 ] ) == 4 );

	assert( solve( 3, [ 7, 3, 8, 1, 4, 6, 2, 5, 9 ] ) == 4 );
	assert( solve( 3, [ 7, 3, 8, 4, 6, 1, 2, 5, 9 ] ) == 4 );
	assert( solve( 3, [ 7, 3, 8, 4, 1, 6, 2, 5, 9 ] ) == 4 );
}
