> #include<stdio.h>
> #include<string.h>
> #define Size 500000
> int a[Size+5];
> char exist[Size*10];
> int main()
> {
> 	int i,k;
> 	a[0]=0;
> 	memset(a,0,sizeof(exist));
> 	exist[0]=1;
> 	for(i=1;i<=Size;i++)
> 	{
> 		if(a[i-1]-i<=0||exist[a[i-1]-i]==1)
> 			a[i]=a[i-1]+i;
> 		else
> 			a[i]=a[i-1]-i;
> 		exist[a[i]]=1;
> 	}
> 	while(scanf("%d",&k),k!=-1)
> 		printf("%d\n",a[k]);
> 	return 0;
> }

挺好...