#include <iostream>

using namespace std;

const int MAX_CASES = 1000, INFINITE=1000*1000*1000;

int values[MAX_CASES];
int movesTo[MAX_CASES];
int nbCases = 0;

void pathfinder(int pos, int curMv)
{
    if(pos < 0 || pos >= nbCases || curMv >= movesTo[pos] || values[pos] == -1)
        return;
    movesTo[pos] = curMv;

    pathfinder(pos-1,curMv+1);
    pathfinder(pos+1,curMv+1);
    pathfinder(values[pos],curMv+1);
}

int main(void)
{
    cin >> nbCases;
    for(int pos=0; pos<nbCases; pos++)
    {
        movesTo[pos] = INFINITE;
        cin >> values[pos];
    }

    if(values[nbCases-1] == -1)
        cout << "-1\n";
    else
    {
        pathfinder(0,0);
        if(movesTo[nbCases-1] == INFINITE)
            cout << "-1\n";
        else
            cout << movesTo[nbCases-1] << "\n";
    }
/*
   for(int pos=0; pos<nbCases; pos++)
        cout << movesTo[pos] << "\t";
    cout << "\n";
*/
    return 0;
}