#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    int N;
    cin >> N;
    vector<int> A(N);
    for ( int i = 0; i < N; i++ ) {
        cin >> A[i];
    }
    
    int x = 0;
    for ( int i = N-1; i >= 0; i-- ) {
        if ( A[i] != i+1 ) {
            x = i;
            break;
        }
    }
    
    if ( A[0] == x ) { x--; }
    
    cout << x << endl;
    
    return 0;
}