Codility Frog River One

[Westerner]

Heya, I've been running through Codility challenges and came upon a snag.
This program is meant to output the shortest time (K) possible to cross the river (every integer between 0 and X + 1 is covered by a leaf); it runs at 100% correctness but fails on all performance tests.
I'm at a loss as to how to increase the speed here:

def solution(X, A) :
    K = 0
    river = list()
        #UNUSABLE-LEAVES-BE-GONE
    for leaf in range(len(A)) :
        #DUPES-BE-GONE
        if A[K] in river:
            K = K + 1
            continue
        #BUILD A BRIDGE
        else :
            river.append(A[K])
        #BRIDGE IS DONE
        if len(river) == X :
            break
        #HERE BE AN ITERATION INCREMENT
        K = K + 1
        #BRIDGE WAS SUCCESSFUL
    if K < len(A) :
        print('tick',K,'out of:',len(A) - 1)
        #BRIDGE WAS A FLOP
    else :
        print(-1)
    pass

Thanks for any tips! (: