#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++){
        scanf("%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);
    return 0;
}