求和

/*
*功能:判断一个数是否是素数
*日期:2012-9-1
*/

# include <stdio.h>
int main(){

    int sum = 0;
    int t = -1;
    int a;
    int i;
    for (i = 0; i<=2001 ;i++ )
    {
        a = 2 * i + 1;
        sum = sum + (-a * t);
        t = -t;
    }
    printf ("sum = %d \n", sum);

}