/** made by amunduzbaev **/
#include <bits/stdc++.h>
//~ #include <ext/pb_ds/assoc_container.hpp>
//~ #include <ext/pb_ds/tree_policy.hpp>
//~ #include <ext/rope>
//~ using namespace __gnu_pbds;
//~ using namespace __gnu_cxx;
using namespace std;
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define ub upper_bound
#define lb lower_bound
#define ll long long
#define ld long double
#define pii pair<int, int>
#define pll pair<ll, ll>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(),x.rend()
#define fastios ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define vll vector<ll>
#define vii vector<int>
#define vpii vector<pii>
#define vpll vector<pll>
#define cnt(a)__builtin_popcount(a)
template<class T> bool umin(T& a, const T& b) {return a > b? a = b, true:false;}
template<class T> bool umax(T& a, const T& b) {return a < b? a = b, true:false;}
//~ typedef tree<pii,null_type,less<pii>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
const int N = 1e3+5;
const int mod = 1e9+7;
const ll inf = 1e18;
const ld Pi = acos(-1);
int s, n, m, k, t, a[N], used[N], tt[N];
vii ans;
void solve(){
cin>>n;
for(int i=1;i<=n;i++) cin>>a[i];
for(int i=1;i<=n;i++){
tt[1] = i;
used[i] = 1;
for(int j = 1, cnt = 2;j<=n;j++) {
if(used[j]) continue;
tt[cnt++] = j;
}
memset(used, 0, sizeof used);
used[a[1]] = 1;
bool ok = 1;
for(int j=2;j<=n;j++){
if(a[j] == tt[j]) continue;
if(used[tt[j]]){
used[a[j]] = 1;
continue;
}else if(tt[j] != a[j]){ ok = 0; break; }
}
if(ok) ans.pb(i);
memset(used, 0, sizeof used);
}
if(sz(ans) > 1) cout<<0<<"\n";
else cout<<ans[0]<<"\n";
ans.clear();
return;
}
int main(){
fastios
int t = 0;
if(t) solve();
else {
cin>>t;
while (t--) solve();
}
return 0;
}
