#!/bin/python

def solve():
    num = int(input())
    array = list(map(int, input().split()))
    if num == 1:
        print(1)
        return

    res = array[num - 1] - array[num - 2] 

    for i in range(num - 1, 0, -1):
        if (array[i] - array[i-1] != res):
            print(i + 1)
            return
    
    print(1)


def main():
    try:
        while True:
            solve()
    except:
        pass

main()