#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int N,K;
int A[202020];

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>K;
	FOR(i,N) cin>>A[i+1];
	
	ll ret=A[K];
	if(A[K]==1) {
		ll L=0,R=0;
		for(i=K+1;i<=N;i++) {
			R+=A[i];
			if(A[i]<=1) break;
		}
		for(i=K-1;i>=1;i--) {
			L+=A[i];
			if(A[i]<=1) break;
		}
		ret+=max(L,R);
	}
	else if(A[K]>=2) {
		for(i=K+1;i<=N;i++) {
			ret+=A[i];
			if(A[i]<=1) break;
		}
		for(i=K-1;i>=1;i--) {
			ret+=A[i];
			if(A[i]<=1) break;
		}
	}
	
	cout<<ret<<endl;
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}