#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) {
            print("Invalid input");
            return 0;
        }
        sum += arr[i];
    }
    total = n * (n + 1) / 2;
    printf("%d",total - sum);
    
    return 0;
}