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