求1-12+13-14+15-16+…1n用while循环

#include
int main()
{
    int a,b,n;
    double m,i,sum;
    a = 0;b = 1;
    m = 1.0;i = 1.0;sum = 0.0;
    scanf("%d",&n);
    while(a < n)
    {
         sum += m;
         a++;
         b = -b;
         i++;
         m = b/i;
    }
    printf("%5.2f\n",sum);
    return 0;
}

希望大家提出好的建议，感谢！