#include<iostream>
using namespace std ;
int main(){
    try{
        int num,in;
        cin>>num;
        int arr[num];
        cin>>in;
        for(int i =0;i<num;i++){
            cin>>arr[i];
        }
        if(arr[i]==arr[in]){
                cout<<arr[in];
            }
            else{
                throw "-1";
            }
        
    }
    catch(const char* msg){
        cout<<msg;
        }
}