read n
for((q=w=1;r<w;++i<n?p[i]=t=i&1?t+1:p[i/2]:z[r<m||(m=w,++v),a=q[r++]]|a>n||(z[a]=v$f$f*-1))){
f=,q[w++]=a+p[a]
}
echo ${z[n]--1}