import java.io.IOException; import java.io.InputStream; import java.util.NoSuchElementException; public class Main { FastScanner fs = new FastScanner(); public static void main(String[] args) { new Main().solve(); } void solve() { int N = fs.nextInt(); int Width = fs.nextInt(); int[] a = new int[N]; int Count = 0; for(int i = 0; i < N; i++) { a[i] = fs.nextInt(); } if(a.length == 1) { System.out.println(0); return; } int i = 0; int loopCount = 0; if((i + Width) < N) { while (true) { if (a[i] > a[i + Width]) { int Temp = a[i + Width]; a[i + Width] = a[i]; a[i] = Temp; Count++; loopCount++; } i++; if (loopCount == 0) { break; } if (i == (a.length - Width)) { i = 0; loopCount = 0; } } } for(int j = 0; j < N; j++) { if(a[j] > a[j + 1]) { System.out.println(-1); return; } } System.out.println(Count); } } class FastScanner { private final InputStream in = System.in; private final byte[] buffer = new byte[1024]; private int ptr = 0; private int buflen = 0; private boolean hasNextByte() { if (ptr < buflen) { return true; } else { ptr = 0; try { buflen = in.read(buffer); } catch (IOException e) { e.printStackTrace(); } if (buflen <= 0) { return false; } } return true; } private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1; } private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126; } private void skipUnprintable() { while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; } public boolean hasNext() { skipUnprintable(); return hasNextByte(); } public String next() { if (!hasNext()) throw new NoSuchElementException(); StringBuilder sb = new StringBuilder(); int b = readByte(); while (isPrintableChar(b)) { sb.appendCodePoint(b); b = readByte(); } return sb.toString(); } public long nextLong() { if (!hasNext()) throw new NoSuchElementException(); long n = 0; boolean minus = false; int b = readByte(); if (b == '-') { minus = true; b = readByte(); } if (b < '0' || '9' < b) { throw new NumberFormatException(); } while (true) { if ('0' <= b && b <= '9') { n *= 10; n += b - '0'; } else if (b == -1 || !isPrintableChar(b)) { return minus ? -n : n; } else { throw new NumberFormatException(); } b = readByte(); } } public int nextInt() { if (!hasNext()) throw new NoSuchElementException(); int n = 0; boolean minus = false; int b = readByte(); if (b == '-') { minus = true; b = readByte(); } if (b < '0' || '9' < b) { throw new NumberFormatException(); } while (true) { if ('0' <= b && b <= '9') { n *= 10; n += b - '0'; } else if (b == -1 || !isPrintableChar(b)) { return minus ? -n : n; } else { throw new NumberFormatException(); } b = readByte(); } } }