#include<stdio.h>
int main(){
    int size,result;
    scanf("%d",&size);
    int arr[size-1];
    for(int i =0;i<size-1;scanf("%d ",&arr[i]));
    int arr1[size];
    for(int j=0;j<size;j++){
        arr[i]=j;
    }
    for(int k=0;k<size;k++){
        if(arr[k]!=arr1[k]){
            result=arr1[k];
            break;
        }
    }
    printf("%d",result);
    return 0;
}