# include<iostream> # include<cstring> # include<cmath> using namespace std; int main() { int n; cin>>n; int a,b[1001]={0}; for(int i=1;i<=n;i++){ cin>>a; b[a]++; } for(int i=1;i<=1000;i++){ if(b[i]==1){ cout<<i;} } return 0; }