#include<bits/stdc++.h>
using namespace std;
int n[1000000],ans;
int main(){
	int a,b;
	cin>>a>>b;
	for(int i=0;i<a;i++){
	    cin>>n[i];
	    if(i+1==b) a[i]==-a[i];
	    ans+=a[i];
	}
	cout<<ans;
}