#include <bits/stdc++.h>
#include <iostream>
using namespace std;
int main(void){
    int N,K;
    cin >> N >> K;
    vector<long> A(N);
    vector<long> ASUM(N);
    
    int startidx=0;
    int endidx=N-1;
    
    for(int i=0;i<N;i++){
        if(i!=K-1){
            long s;
            cin >> s;
            A.at(i)=s;
            if(i==0){
                ASUM.at(i)=s;
            }else{
                ASUM.at(i)=ASUM.at(i-1)+s;
            }
            if(s<=1&&i<K-1){
                startidx=i;
            }else if(s<=1&&i>K-1){
                endidx=i;
            }
        }else{
            cin >> A.at(i);
            if(i>0){
                ASUM.at(i)=ASUM.at(i-1)+A.at(i);
            }else{
                ASUM.at(i)=A.at(i);
            }
        }
    }

    if(A.at(K-1)==0){
        cout << 0 << endl;
    }else if(A.at(K-1)==1){
        long ans1=ASUM.at(K-1)-ASUM.at(startidx);
        if(startidx==0){
            ans1+=ASUM.at(0);
        }
        long ans2=ASUM.at(endidx)-ASUM.at(K-1);
        if(ans1<ans2){
            cout << ans2 << endl;
        }else{
            cout << ans1 << endl;
        }
    }else{
        if(startidx>0){
            cout << ASUM.at(endidx)-ASUM.at(startidx-1) << endl;
        }else{
            cout << ASUM.at(endidx) << endl;
        }
    }
    
}