First-Unique

from collections import defaultdict
def solution(A):
    cnts = defaultdict(int)
    idx = {}

    for i,n in enumerate(A):
        cnts[n] += 1
        if n not in idx:
            idx[n] = i
    
    i = float('inf')
    for k,v in filter(lambda kv: kv[1] == 1, cnts.items()):
        i = min(i, idx[k])
    return A[i] if i != float('inf') else -1