// editor2
int main(){
    int N,num;
    int expected_sum ,actual_sum = 0;
    scanf("%d",&N);
    if(N<0 || N>10){
        printf("Invalid input");
        return 0;
    }
    expected_sum = N * (N+1)/2;
    for(int i=0; i<N-1; i++){
        scanf("%d",&num);
        actual_sum +=num;
    }
    printf("%d", expected_sum - actual_sum);
    return 0;
    
}