为什么这个代码是对的

#include <iostream>
#define ll long long
#define endl "\n"
using namespace std;
const int N = 2e5 + 5;
int n, r, nxt, per, num0, num1, a[N];
int main()
{
    cin >> n >> r;
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    nxt = 1, per = n;
    while (a[nxt] == 1 && nxt <= r)
        nxt++;
    while (a[per] == 1 && per > r)
        per--;
    for (int i = nxt; i <= per; i++)
        (a[i] ? num1 : num0)++;
    if (nxt == per)
        cout << 0 << endl;
    else
        cout << num0 + num1 * 2 << endl;
    return 0;
}