#include <bits/stdc++.h>

using namespace std;

int main () {
	ios::sync_with_stdio(false);
    
    int n;
    
    while ( cin >> n ) {
        long long prev, dif = 0, start;
        cin >> prev;
        for ( int i = 1; i < n; ++i ) {
            long long x;
            cin >> x;
            
            if ( prev + dif != x ) {
                dif = x - prev;
                start = i;
            }
            
            prev = x;    
        
        }
    	cout << start << endl;
    
    
    }

	return 0;
}