#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>;

unordered_map<ll,ll> nx[1007];

int main(){
  ios::sync_with_stdio(false);cout.tie(0);cin.tie(0);
  for(;;) {
    ll n;cin >>n;
    if(!cin)break;
    F(n)nx[i].clear();
    vl v(n);
    F(n) cin>>v[i];
    ll mx=0; ll mxi=-1;
    for(ll i=n-1; i>=0; i--) {
      for(ll j=n-1; j>i; j--) {
        if(j==n-1) { nx[i][v[j]-v[i]]=1; if(mx<=1) { mx=1;mxi=i; } }
        else {
          ll df=v[j]-v[i];
//          dout<<i<<' '<<j<<' '<<df<<endl;
          if(nx[j][df] && nx[i][df]<nx[j][df]+1)
            nx[i][df]=nx[j][df]+1;
          if(nx[i][df] >= mx) { mx=nx[i][df]; mxi=i; }
        }
      }
    }
    cout<<mxi+1<<endl;
  }
  return 0;
}