#include<bits/stdc++.h>
using namespace std;

bool binarysearch(int arr[], int n,int key)
{
    int s=0;
    int e=n-1;
    while(s<=e)
    {
        int mid=(s+e)/2;

        if(arr[mid]>key){
            e=mid-1;
        }
        if(arr[mid]<key){
            s=mid+1;
        }
        if(arr[mid]==key)
        {
            return true;
        }
        else{
            return false;
        }

    }
}

int main(){
    int n;
    cin>>n;

    int arr[n-1];
    for (int i = 0; i < n-1; i++)
    {
        cin>>arr[i];
    }

    sort(arr,arr+n);
    for (int i = 0; i < n; i++)
    {
        if(binarysearch(arr,n,i)){
            continue;
        }
        else{
            cout<<i<<endl;
            break;
        }
    }



return 0;
}