import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

/*
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/**
 *
 * @author cteam014
 */
public class Amusement {
    
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer tokenizer;
    
    int difference;
    int N;
    int[] sequence;

    
    public void solve() throws  Exception {
        String line;
        while ((line = reader.readLine()) != null) {
            N = Integer.parseInt(line);
            
            if (N < 2) {
                reader.readLine();
                System.out.println(1);
                continue;
            }
            
            sequence = new int[N + 1];
            
            tokenizer = new StringTokenizer(reader.readLine());
            for (int i = N; i > 0; i--) {
                sequence[i] = Integer.parseInt(tokenizer.nextToken());
            }
            
            difference = sequence[1] - sequence[2];
            int i = 2;
            while (i < sequence.length - 1 && sequence[i] - sequence[i + 1] == difference) {
                i++;
            }
            System.out.println(N - i + 1);
        }   
    }
    
    public static void main(String[] args) throws Exception {
        new Amusement().solve();
    }
    
}