import std.stdio, std.array, std.string, std.conv, std.algorithm;
import std.typecons, std.range, std.random, std.math, std.container;
import std.numeric, std.bigint, core.bitop, core.stdc.stdio;

void main() {
    auto N = readln.chomp.to!int;
    auto A = readln.split.map!(to!int).array;

    int cnt = 1;
    foreach (a; A) {
        if (cnt == a) {
            cnt += 1;
        }
    }

    int ans = N - cnt + 1;


    writeln(cnt == 1 ? -1 : ans);
}