#include <iostream>
#include <vector>

using namespace std;

/*int main() {
    long n,d;
    long v[1000];
    while (cin >> n) {
        if (n <= 2) {
            cout << "1" << endl;
            continue;
        }

        for (int i=0;i<n;i++) {
            cin >> v[i];
        }
        d = v[n-1] - v[n-2];
        int i = n-2;
        for (;i>=0;i--) {
            if (v[i]+d != v[i+1]) {
                break;
            }
        }
        cout << i+2 << endl;
    }
    return 0;
}
*/

int main() {
    int n;
    while(cin >> n) {
        vector<int> v(n);
        for(int i = 0; i<n; i++) cin >> v[i];
        int i=n-3;
        for(; i>=0; i--) {
            if(v[i]-v[i+1] != v[i+1]-v[i+2])
                break;
        }

        if(i<0) i=-1;
        i+=2    ;
        cout << i << endl;
    }
}