#include void main(){int i,n;double d=-1,k,s=1;scanf("%d",&n);for(i=1;i<=n;i++){k=d/(i*(i+1));s+=k;d=-d;}printf("%lf",s);}
=1/（n+1）-1/n+1/n-1/(n-1)...+1=1/(n+1)
