def find_even_index(arr):
    for i in range(len(arr)):
        if sum(arr[:i]) == sum(arr[i+1:]):
            return i
    return -1

# Test.assert_equals(find_even_index([1,2,3,4,3,2,1]),3)
# Test.assert_equals(find_even_index([1,100,50,-51,1,1]),1,)
# Test.assert_equals(find_even_index([1,2,3,4,5,6]),-1)
# Test.assert_equals(find_even_index([20,10,30,10,10,15,35]),3)
# Test.assert_equals(find_even_index([20,10,-80,10,10,15,35]),0)
# Test.assert_equals(find_even_index([10,-80,10,10,15,35,20]),6)
# Test.assert_equals(find_even_index(range(1,100)),-1)
# Test.assert_equals(find_even_index([0,0,0,0,0]),0,"Should pick the first index if more cases are valid")
# Test.assert_equals(find_even_index([-1,-2,-3,-4,-3,-2,-1]),3)
# Test.assert_equals(find_even_index(range(-100,-1)),-1)