#include <bits/stdc++.h>
using namespace std;

#define FOR(i,a,b) for(auto i=a;i<b;++i)
#define F(a) FOR(i,0l,a)
#define FF(a) FOR(j,0l,a)
#define aa first
#define bb second
#define PB push_back
#define pb push_back
#define MOD ((int)1e9+7)
#define INF (1<<30)
#define LINF (1L<<61)
#define EPS (1e-10)
#define EQ(a,b) (fabs(a-b)<=(fabs(a+b)*EPS))
#ifdef DEBUG
#define DEB puts("DEB!")
#define DPRINT(X) printf("#X: %d\n",X)
#define dout cout
#else
#define DEB do{}while(0)
#define DPRINT(X) do{}while(0)
#define dout 0 && cout
#endif
using ll=long long;
using ull=unsigned long long;
using ld=double;
using vl=vector<int>;
using vvi=vector<vl>;
using pii=pair<int,int>;

int main(){
  ios::sync_with_stdio(false);
  for(;;) {
    ll n;cin >>n;
    if(!cin)break;
    vl v(n);
    F(n)cin>>v[i];
    ll i=n-1;
    while(i-1>=0 && (i+1==n || v[i-1]-v[i] == v[i]-v[i+1])) i--;
    cout<<i+1<<endl;
  }
  return 0;
}