#include<bits/stdc++.h>

#define ll long long
#define ALL(v) (v).begin(),(v).end()
#define REP(i,p,n) for(int i=p;i<(int)(n);++i)
#define rep(i,n) REP(i,0,n)
#define dump(a) (cerr << #a << "=" << (a) << endl)
#define DUMP(list) cout << "{ "; for(auto nth : list){ cout << nth << " "; } cout << "}" << endl;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }


using namespace std;

int main(){

    int N, K;
    cin >> N >> K;
    int l = 0;
    int r = N-1;
    ll A[N];
    rep(i, N) {
        cin >> A[i];
        if (A[i] <= 1 && i < K - 1) {
            l = i;
        } else if (A[i] <= 1 && i > K - 1 && r == N-1) {
            r = i;
        }
    }

    ll ans1 = 0;
    ll ans2 = 0;
    REP(i, l, K) {
        ans1 += A[i];
    }

    REP(i, K, r+1) {
        ans2 += A[i];
    }

//    cout << ans1 << " " << ans2 << endl;

    if (A[K - 1] == 0) {
        cout << 0 << endl;
    } else if (A[K - 1] == 1) {
        cout << max(ans1 , ans2+1) << endl;
    } else {
        cout << ans1 + ans2 << endl;
    }

}