#include <bits/stdc++.h>
#include <algorithm>
using namespace std;
 
#define C continue;
#define R return
 
#define I insert
#define D double
 
#define ull unsigned long long int
#define ll long long int
#define ui unsigned int
#define ld long double
#define pb push_back
#define pf push_front
 
#define vi vector<int>
#define vc vector<char>
#define vs vector<string>
#define vb vector<bool>
#define vd vector<D>
#define vll vector<ll>
#define vull vector<ull>
#define vld vector<ld>
 
#define vvi vector<vector<int>>
#define vvb vector<vector<bool>>
#define vvc vector<vector<char>>
#define vvll vector<vector<ll>>
#define vvd vector<vector<D>>
#define vvld vector<vector<ld>>
 
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pull pair<ull, ull>
#define pld pair<ld, ld>
#define pdd pair<D, D>
#define psi pair<string, int>
#define pis pair<int, string>
 
#define vpld vector<pld>
#define vpii vector<pii>
#define vpll vector<pll>
#define vpdd vector<pdd>
 
#define vvpii vector<vector<pii>>
#define vvpll vector<vector<pll>>
 
#define PQ(type) priority_queue<type>
#define PQD(type) priority_queue<type, vector<type>, greater<type>>
 
#define si set<int>
#define sll set<ll>
#define ss set<string>
#define sc set<char>
#define sd set<double>
 
#define mii map<int, int>
#define mll map<ll, ll>
#define mci map<char, int>
#define msi map<string, int>
#define mis map<int, string>
 
#define umii unordered_map<int, int>
#define umll unordered_map<ll, ll>
#define umci unordered_map<char, int>
#define umsi unordered_map<string, int>
#define umis unordered_map<int, string>
 
#define dist(a, b, p, q) sqrt((p - a) * (p - a) + (q - b) * (q - b))
 
#define pp(n) printf("%.10Lf", n);
#define line cout << "\n";
 
#define mod1 1000000007
#define mod2 998244353
#define test  \
    int t;    \
    cin >> t; \
    while (t--)
#define init(arr, val) memset(arr, val, sizeof(arr))
#define loop(i, a, b) for (int i = a; i < b; i++)
#define loopr(i, a, b) for (int i = a; i >= b; i--)
#define loops(i, a, b, step) for (int i = a; i < b; i += step)
#define looprs(i, a, b, step) for (int i = a; i >= b; i -= step)
#define P pair
#define F first
#define S second
#define pb push_back
#define pf push_front
#define MM multimap
#define fast                          \
    ios_base::sync_with_stdio(false); \
    cin.tie();                        \
    cout.tie();
#define read freopen("input.txt", "r", stdin);
#define write freopen("output.txt", "w", stdout);
#define endl "\n"
#define f(i, n) for (int i = 0; i < n; i++)
#define ff(i, n) for (int i = 1; i <= n; i++)
#define fr(i, n) for (int i = n - 1; i >= 0; i--)
#define pb push_back
#define eb emplace_back
#define bgn(x) ((x).begin())
#define sz(x) ((x).size())
#define in1(x, n) f(i, n) cin >> x[i];
#define in2(x, n, m) f(i, n) f(j, m) cin >> x[i][j];
#define out1(x, n)               \
    f(i, n) cout << x[i] << " "; \
    cout << "\n";
#define out2(x, n, m)                   \
    f(i, n)                             \
    {                                   \
        f(j, m) cout << x[i][j] << " "; \
        cout << "\n";                   \
    }
#define fir(x, y) f(int i = min(x, y); i <= max(x, y); i++)
#define firr(x, y) f(int i = max(x, y); i <= min(x, y); i++)
#define imin INT_MIN
#define imax INT_MAX
#define lmax LLONG_MAX
#define lmin LLONG_MIN
#define dmin DBL_MIN
#define dmax DBL_MAX
 
#define mxt(a) *(max_element(a.begin(), a.end()))
#define mnt(a) *(min_element(a.begin(), a.end()))
#define mem(x, val) memset(x, val, sizeof(x))
#define prdbl(val, precise) cout << fixed << setprecision(precise) << val << "\n";
#define dbg1(x) cout << " > " << #x << " = " << x << "\n";
#define dbg2(x, y) cout << " < " << #x << " = " << x << ", " << #y << " = " << y << endl;
#define count_1(n) __builtin_popcountll(n)
#define sqr(x) ((x) * (x))
#define inrange(i, a, b) ((i >= min(a, b)) && (i <= max(a, b)))
#define lb lower_bound
#define ub upper_bound
#define rev reverse
const ll maxn = 1e5 + 5;
const ll inf = 1e9;
const long double PI = acos(-1);
 
void yes() { cout << "YES";
    cout << "\n";
}
void no() { cout << "NO";
    cout << "\n";
}
ll pmod(ll a, ll mod = mod1) { return ((a) % mod + mod) % mod; }
ll cel(ll x1, ll y1) //not for negative x1
{
    if ((x1) <= 0)
    {
        return (x1) / (y1);
    }
    return (((x1) / (y1)) + (((x1) % (y1)) != 0));
}
 
ll gcd(ll a, ll b)
{
    if (b == 0)
        return a;
    else
        return gcd(b, a % b);
}
 
ll lcm(ll a, ll b)
{
    return (a / gcd(a, b)) * b;
}
 
long long bp(long long a, long long b)
{
    long long res = 1;
    while (b > 0)
    {
        if (b & 1)
            res = res * a;
        a = a * a;
        b >>= 1;
    }
    return res;
}
 
long long bpm(long long a, long long b, long long m = mod1)
{
    if (a >= m)
        a %= m;
    long long res = 1;
    while (b > 0)
    {
        if (b & 1)
            res *= a;
        if (res >= m)
            res %= m;
        a *= a;
        if (a >= m)
            a %= m;
        b >>= 1;
    }
    if (res >= m)
        res %= m;
    return res;
}
 
string vow = "aeiou";
 
const int dx4[] = {-1, 0, 1, 0};
const int dy4[] = {0, 1, 0, -1};
 
const int dxk[] = {-1, 0, 1, 0, -1, -1, 1, 1};
const int dyk[] = {0, 1, 0, -1, -1, 1, -1, 1};
 
const int dxh[] = {-2, -2, -1, -1, 1, 1, 2, 2};
const int dyh[] = {1, -1, 2, -2, 2, -2, 1, -1};
 
 
void solve()  
{
    int n;
    cin >> n;
    vi arr(n);
    in1(arr, n);
    int cnt = 0;
    f(i,n){
       if(arr[i]==cnt+1)
           cnt++;
    }
    if(cnt)
        cout << n - cnt;
        else
            cout << -1;
}
int main()
{
    //read
    //write
    fast;
    int t = 1;
    //cin >> t;
    while (t--)
    {
 
        solve();
        line
    }
 
    return 0;
}

Measured memory_bytes: 1024