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