#include <iostream>

using namespace std;

int main(int argc, char** argv) {

    unsigned int N = 0;
    unsigned int index = 0;
    long long first = 0, second = 0;
    long long d = 0;
    
    while (cin >> N) 
    {
        cin >> first;
        for (unsigned int i = 0; i < N - 1; i++) 
        {
            cin >> second;
            
            if (first + d != second) {
                d = second - first;
                index = i;
            }
            
            first = second;
        }
        cout << index + 1 << endl;
    }
    
    return 0;
}