#include <bits/stdc++.h>

#define int long long

using namespace std;

using i128 = __int128;

#define lep(i, l, r) for(int i = (l); i <= (r); i ++)
#define rep(i, l, r) for(int i = (l); i >= (r); i --)
#define Lep(i, l, r) for(int i = (l); i <  (r); i ++)
#define debug(...) fprintf (stderr, __VA_ARGS__)

using i64 = long long;
using ld = long double;
using ui64 = unsigned long long;

const int N = 2e5 + 5;

int n, x;
int a[N];
vector<int> e[N];

void solve() {
   cin >> n >> x;
	lep (i, 0, n - 1) cin >> a[i];
	lep (i, 1, n - 1) {
		int to = i + a[i];
		to = to % n;
		e[i].push_back(to);
	}
	
	e[0].push_back(n), e[n].push_back((a[0] + 1) % n);
	
	lep (i, 1, n - 1) {
		int to = i + 1;
		if (to == n) to = 0;
		e[i].push_back(to);
	}

	vector<int> dis(n + 1, -1);
	queue<int> q;
	q.push(0);
	dis[0] = 0;
	while (!q.empty()) {
		int u = q.front(); q.pop();
		for (int v : e[u]) {
			if (dis[v] == -1) {
				dis[v] = dis[u] + 1;
				q.push(v);
			}
		}
	}
	if (dis[x] == -1) cout << -1 << endl;
	else cout << dis[x] << endl;
}

signed main() {
	ios :: sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	int Case = 1;;
	//cin >> Case;
	while (Case --) solve();
	return 0;
}