的Windows線程API：計算與多線程

我目前在做這個項目，我需要計算圓周率值PI值...的Windows線程API：計算與多線程

當指定只有一個線程完美的作品，我也得到3.1416 [ ...]但是當我指定解決過程中的2級或更多的線程我停下得到3.1416價值，這是我的代碼：

#include <stdio.h> 
#include <time.h> 
#include <windows.h> 

//const int numThreads = 1; 
//long long num_steps = 100000000; 

const int numThreads = 2; 
long long num_steps = 50000000; 

double x, step, pi, sum = 0.0; 
int i; 

DWORD WINAPI ValueFunc(LPVOID arg){ 
    for (i=0; i<=num_steps; i++) { 
     x = (i + .5)*step; 
     sum = sum + 4.0/(1. + x*x); 
    } 

    printf("this is %d step\n", i); 
    return 0; 
} 

int main(int argc, char* argv[]) { 
    int count; 
    clock_t start, stop; 
    step = 1./(double)num_steps; 
    start = clock(); 

    HANDLE hThread[numThreads]; 
    for (count = 0; count < numThreads; count++) { 

      printf("This is thread %d\n", count); 
     hThread[count] = CreateThread(NULL, 0, ValueFunc, NULL, 0, NULL); 

    } 

    WaitForMultipleObjects(numThreads, hThread, TRUE, INFINITE); 

    pi = sum*step; 
    stop = clock(); 
    printf("The value of PI is %15.12f\n", pi); 
    printf("The time to calculate PI was %f seconds\n", ((double)(stop - start)/1000.0)); 

}

指定2個線程，當我得到這個錯誤輸出：

Wrong PI value