a[110],x[200010],i,j;main(t){for(*x=1;~scanf("%d",a+j++););for(;i<*a;i++)for(j=1;x[i]&&a[++j];)!x[t=i+a[j]]|x[t]>x[i]?x[t]=x[i]+1:0;i=!printf("%d",x[*a]-1);}