（C++）利用下列公式編程計算π的值？

#include <iostream>

using namespace std;

int main()

{

float sum = 0;

float pai;

for(int n=1;n<=2000;n++)

{

float a1 = 1.0 / (4*n-3);

float a2 = 1.0 / (4*n-1);

sum = sum + a1 - a2;

}

pai = sum * 4;

cout << "π=" << pai << endl;

return 0;

}