#include <cstdio>
#include <set>

#define X first
#define Y second

using namespace std;

typedef pair < int, int > pii;
typedef long long ll;
typedef pair < ll, pii > plp;

const int N = 1e5 + 500;

int a[N], n, dst, bio[N];
ll ans[N];

set < plp > S;
set < pii > tme;

void add_closest(int x){
	if(x == -1 || tme.empty()) return;
	int y = tme.rbegin() -> Y;
	if(tme.begin() -> X <= x) {
		y = (--tme.lower_bound({x, -1}))->Y;
	}
	S.insert({1 + ans[x] + (x + 2 * n - a[y] - y) % n, {y, x}});
	// ubaci y
}

int main(){
	scanf("%d%d", &n, &dst);
	for(int i = 0;i < n;i++) {
		scanf("%d", a + i);	
		if(i != dst) {
			tme.insert({(i + a[i]) % n, i});
			ans[i] = -1;
		} else {
			S.insert({0, {i, -1}});
			ans[i] = 0;
		}
	}
	for(;!S.empty();) {
		int st = S.begin()->Y.X;
		int from = S.begin()->Y.Y;
		if(bio[st]) {
			add_closest(from);
			S.erase(S.begin());
			continue;
		}
		ans[st] = S.begin()->X;
		S.erase(S.begin());
		bio[st] = 1;
		tme.erase({(st + a[st]) % n, st});
		add_closest(from);
		add_closest(st);
	}
	printf("%lld\n", ans[0]);
	return 0;
}