#include <bits/stdc++.h>

using namespace std;

typedef long long int ll;
typedef double ld;

typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<ii> vii;

#define PB push_back
#define FOR(prom, a, b) for ( ll prom = (a); prom < (ll)(b); ++prom )
#define F(a) FOR(i,0,a)
#define FF(a) FOR(j,0,a)

#define EPS (1e-10)

int main () {
	ios::sync_with_stdio(false);
	int N;
	while(cin>>N){
		vector<int> a(N),b(N-1);
		F(N)cin>>a[i];
		F(N-1)b[i]=a[i+1]-a[i];
		int res = N-1;
		int check = b[N-2];
		for(int i = N-2; i>=0; --i){
			if(check!=b[i])break;
			res = i + 1;
		}
		/*F(N)cout<<a[i]<<' ';
		cout<<endl;
		F(N-1)cout<<b[i]<<' ';
		cout<<endl;
		cout<<check<<endl;*/
		cout<<res<<endl;
	}
	return 0;
}