#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int,int> P;

#define p_ary(ary,a,b) do { cout << "["; for (int count = (a);count < (b);++count) cout << ary[count] << ((b)-1 == count ? "" : ", "); cout << "]\n"; } while(0)
#define p_map(map,it) do {cout << "{";for (auto (it) = map.begin();;++(it)) {if ((it) == map.end()) {cout << "}\n";break;}else cout << "" << (it)->first << "=>" << (it)->second << ", ";}}while(0)

template<typename T1,typename T2>ostream& operator<<(ostream& os,const pair<T1,T2>& a) {os << "(" << a.first << ", " << a.second << ")";return os;}

int main() {
    int n,k;
    cin >> n >> k;
    vector<int> a(n);
    for (int i = 0;i < n;++i) cin >> a[i];
    k--;
    ll ans = a[k];
    if (a[k]) {
        ll l = 0,r = 0;
        for (int i = k-1;i >= 0;--i) {
            l += a[i];
            if (a[i] < 2) break;
        }
        for (int i = k+1;i < n;++i) {
            r += a[i];
            if (a[i] < 2) break;
        }
        if (a[k] > 1) ans += l+r;
        else ans += max(l,r);
    }
    cout << ans << endl;
}