#include <iostream>
#include <stdio.h>

using namespace std;

int solveVeryBigProblem(int *& array, int size) {
    
    if(size == 1) return 1;
    
    int j = size - 3, diff = array[size-2] - array[size-1];
    
    for(int i = size -2; j >= 0; i--) {
        
        if(array[j]-array[i] != diff)
            break;
        j--;    
        
    }
    
    return j+2;
}

int main(void) {
   
    int n;
    
    while(scanf("%d", &n) != EOF) {
        
        int * array = NULL, tmp;
        array = new int[n];
        
        for(int i = 0; i < n; i++) {
            scanf("%d", &tmp);
            array[i] = tmp;
        }
        
        cout << solveVeryBigProblem(array, n) << endl;
        
    }

    return 0;
}