var input = require('fs').readFileSync('/dev/stdin', 'utf8');
var lines = input.split("\n");
var n = parseInt(lines[0].split(" ")[0]);
var k = parseInt(lines[0].split(" ")[1]);
var A = lines[1].split(" ");
var a = [];
A.map(function(e) {
        a.push(parseInt(e));
});
var tmp;
var count = 0;

for (var i = 0; i < n; i++) {
        if (i === (n - 1)) {
                count = isSorted(a) ? count : -1; 
                console.log(count);
                process.exit();
        }
        if (a[i] > a[i + k]) {
                tmp      = a[i];
                a[i]     = a[i + k];
                a[i + k] = tmp;
                i = -1;
                count++;
        }
}



function isSorted(arr) {
        for (var i = 0; i < arr.length; i++) {
                if (arr[i] > arr[i+1]) {
                        return false;
                }
        }
        return true;
}