ll@N,n=-N+1,s=0,h=n/2,c=n&1?1:h;REP(k,h+1,n+1)s=10s+(k&1?c:-c),c=c*k*(n-k)/((2k-n)*(2k+1-n));string d(max(n,0),48);wtN(s,"/1",d);