N,M=map(int,input().split())
a=list(map(int,input().split()))

A=[[]for i in range(M)]

for i in range(N):
    A[i%(M)].append(a[i])
    
ans=0
for i in range(M):
    for j in range(len(A[i])-1):
        for k in range(j+1,len(A[i])):
            if A[i][j]>A[i][k]:
                ans+=1

a.sort()
for i in range(M):
    A[i].sort()
B=[]
for i in range(N):
    B.append(A[i%(M)][i//(M)])
if a==B:
    print(ans)
else:
    print(-1)