#include <iostream>
#include <vector>
using namespace std;
int main(){
	int N;
	cin >> N;
	vector<int> a(N);
	for (int i = 0; i < N; i++){
		cin >> a[i];
		a[i]--;
	}
	vector<int> p(N);
	for (int i = 0; i < N; i++){
		p[a[i]] = i;
	}
	for (int i = N - 1; i >= 1; i--){
		if (p[i - 1] >= p[i]){
			cout << i << endl;
			return 0;
		}
	}
	cout << 0 << endl;
}