from sys import stdin

def evalSeq(n, arr):
    if n > 1:
        d = arr[n-2] - arr[n-1]
        for i in range(n-3, -1, -1):
            if arr[i] - arr[i+1] != d:
                return i+2
    return 1


def main():
    check = True
    longA = 0
    arr = []
    while True:
        try:
            line = input()
            if check:
                longA = int(line)
                check = False
            else:
                check = True
                [arr.append(int(x)) for x in line.split()]
                print(evalSeq(longA, arr))
                arr = []
                longA = 0
        except EOFError:
            break

main()