关于openmp的入门程序，大仙们帮忙啊该如何解决

static long num_steps = 10000;double step;double pi;void main(){    int i;    double x,sum = 0.0;        step = 1.0/(double) num_steps;#pragma omp parallel     {#pragma omp for  reduction(+:pi)        for (i=0;i<num_steps;i++)        {            x = (i+0.5)*step;            sum = sum + 4.0/(1.0 +x*x);        }        pi = step * sum;            }    cout <<pi<<endl;}