这个哪里错了，能运行但结果不对

int main() {
    int a, b, i;
    double sum;
    sum = 0;
    i = 1;
    a = 1;
    b = 2;

    do {
        sum += ((double)b) / a;

        a *= 2;
        b += i; 

        i++;
    } while (i <= 10);
    printf("sum=%f", sum);
    return 0;
}

两数相除应该是double而不是int