https://codility.com/demo/results/trainingWXKXMR-EM4/

FrogRiverOne.py

# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"

def solution(X, A):
    # write your code in Python 2.7
    L = len(A)
    P = [-1]*X
    min = 0
    
    for i in range(0, L):
        p = A[i] - 1
        if p < X and P[p] == -1:
                min = i
                P[p] = i
    
    for e in P:
        if e == -1:
            min = e
            break
    
    return min