#include<stdio.h>
 int main(){
     int n,i,x,sum = 0,total =0,invalid =0;
     scanf("%d",&n);
     
     if(n<=0){
         printf("Invalid input");
         return 0;
     }
     
     for(i=1;i<n;i++){
         scanf("%d",&x);
         if(x<0||x>n) invalid= 1;
         sum +=x;
     }
     
     if(Invalid)printf("Invalid input");
     else{
         total = n*(n+1)/2;
         printf("%d",total-sum);
     }
     return 0;
 }