def main():
    
    n, k = map(int, input().split())
    h = list(map(int, input().split()))
    
    '''n = 10
    k = 3
    h = [6, 2, 1, 8, 4, 8, 7, 12, 9, 3]'''


    db = [0]*max(h)
    
    maxi = 0
    for i in range(k-1, -1, -1):
        db[h[i]-1] += 1
        if h[i] > maxi:
            maxi = h[i]-1
    
    if maxi <= h[0]:
        return 0

    for i in range(0, k):
        szam_uj = h[i+k]
        
        db[szam_uj-1] += 1
        if szam_uj > maxi:
            maxi = szam_uj-1
        if maxi+1 <= h[i]:
            return i+1

    for i in range (k, n-k):
        
        szam_kivett = h[i-k]
        szam_uj = h[i+k]
        db[szam_kivett-1] -= 1
        db[szam_uj-1] += 1
        if szam_uj > maxi+1:
            maxi = szam_uj-1
        if szam_kivett >= maxi+1 and db[szam_kivett-1] == 0:
            for j in range(szam_kivett-1, 0, -1):
                if db[j] != 0:
                    maxi = j
        if maxi+1 <= h[i]:
            return i+1

    for i in range(n-k, n):
        szam_kivett = h[i-k]
        db[szam_kivett-1] -= 1
        if szam_kivett >= maxi+1 and db[szam_kivett-1] == 0:
            for j in range(szam_kivett-1, 0, -1):
                if db[j] != 0:
                    maxi = j

        if maxi+1 <= h[i]:
            return i+1
    return -1



print(main())