#include<stdio.h>

int main()
{
  int n,;
  scanf("%d",&n);
  if(n<=0)
  {
      printf("Invalid input");
      return 0;
  }
  int arr[n-1];
  int sum=0;
  for(int i=0;i<n-1;i++)
  {
      scandf("%d"&arr[i]);
      if(arr[i]<=0)
      {
          printf("Invalid input");
          return 0;
      }
      sum+=arr[i];
  }
  int total=n*(n+1)/2;
  int missing=total-sum;
  printf("%d",missing);
        
        
        
        
        
        
        
  
}