#include <bits/stdc++.h>
#define all(v) v.begin(),v.end()
#define eb emplace_back
#define N 1000010
using namespace std;
using ll=long long;
using P=pair<int,int>;
const ll INF=1LL<<30;
inline void chmin(int &a,int b){a=min(a,b);}
int dy[4]={0,-1,0,1};
int dx[4]={1,0,-1,0};
int to[4]={2,3,0,1};
int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	int n,k;cin>>n>>k;--k;
	vector<ll> a(n);
	for(int i=0;i<n;i++){
		cin>>a[i];
	}
	ll l=0,r=0;
	for(int i=k-1;i>=0;i--){
		if(a[i]==0)break;
		if(a[i]==1){
			l++;
			break;
		}else{
			l+=a[i];
		}
	}
	for(int i=k+1;i<n;i++){
		if(a[i]==0)break;
		if(a[i]==1){
			r++;
			break;
		}else{
			r+=a[i];
		}
	}
	if(a[k]==0){
		cout<<0<<endl;
	}else if(a[k]==1){
		cout<<max(l,r)+1LL<<endl;
	}else{
		cout<<l+r+a[k]<<endl;
	}
}