#include <stdio.h>

int main(void)
{
    int n,k,a[1000],s=0,t,i,j;
    scanf("%d%d",&n,&k);
    for(i=0;i<n;++i)scanf("%d",&a[i]);
    for(i=0;i<n-k;++i){
        for(j=n-1;j>=i+k;--j){
            if(a[j-k]>a[j]){
                t=a[j-k];
                a[j-k]=a[j];
                a[j]=t;
                ++s;
            }
        }
    }
    for(i=1;i<n;++i)if(a[i-1]>a[i])break;
    if(i==n)printf("%d",s);
    else puts("-1");
    return 0;
}