#include<stdio.h>
int main()
{
    int n,i,missing;
    scanf("%d",&n);
    int arr[n-1],sum=0,total;
    if(n<0)
    {
        printf("invalid input");
        return 0;
    }
    for(i=0;i<n-1;i++)
    {
        scanf("%d"&arr[i]);
        sum+=arr[i];
    }
    total=n*(n-1)/2;
    missing =total-sum;
    printf("%d",missing);
    return o;
}