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

const int N=2e5+1e3+7;

int n,x,a[N],d[N];

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin>>n>>x;
    for(int i=0;i<n;i++)
        cin>>a[i],d[i]=-1;
    queue<int> q;
    q.push(a[0]);
    d[a[0]]=0;
    while(!q.empty())
    {
        int x=q.front();
        q.pop();
        for(auto v:{(x+a[x])%n,(x+1)%n})
        {
            if(d[v]!=-1)
                continue;
            d[v]=d[x]+1;
            q.push(v);
        }
    }
    cout<<d[x]<<"\n";
}