#include <cstdlib>
#include <vector>
#include<iostream>

using namespace std;


long findIndex(vector<long long>  n){
    long long d = 0;
    long long index=1;
    for ( size_t i = 0 ;  i < n.size(); i++ )
    {
        if ( i == n.size()-1)
            break;
        if ( n[i+1]-n[i] != d )
        {
            d = n[i+1]-n[i];
            index=i+1;
        }
    }
    return index;
}
int main(int argc, char** argv) {
    long long n = 0;
    long long first = 0 ;
    long long second = 0 ;
    while ( cin >> n ){
        vector <long long> numbers; 
        int tmp;
        for ( int i = 0 ; i < n; i++ ){
        cin >> tmp;
        numbers.push_back(tmp);
        }
        cout << findIndex(numbers) << endl;
        
      
    }
    return 0;
}