#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#define N 
#define int long long
#define ui unsigned
#define ul unsigned long long
#define db double
#define fo(i,l,r) for(int i=l;i<=r;i++)
#define rf(i,r,l) for(int i=r;i>=l;i--)
#define mem(x) memset(x,0,sizeof(x));
using namespace std;
int T,n,m,k,ans,op,x,y,z;
void swap(int &x,int &y){int t=x;x=y;y=t;}
int max(int x,int y){return x>y?x:y;}
int min(int x,int y){return x<y?x:y;}
int read(){
	int sum=0;bool f=0;char c=getchar();
	for(;c<48||c>57;c=getchar())if(c==45)f=1;
	for(;c>=48&&c<=57;c=getchar())sum=sum*10+(c&15);
	return f?-sum:sum;
}
signed main(){ 
	freopen("tingche.in","r",stdin);freopen("tingche.out","w",stdout);
	ios::sync_with_stdio(0);
	cin.tie(0);
	n=read();
	fo(i,0,n-1)
		if(i!=read())
			return !printf("%lld",i);
	printf("%lld",n); 
	return 0;
}