为什么错了？求dalao解答

代码：

#include <bits/stdc++.h>
using namespace std;
long long d,x,y,a,b1,b2,mn = 1e12;
int main(){
	cin >> d;
	a = sqrt(d);
	for (int i = 1;i <= a;i++){
		b1 = sqrt(d - i * i),b2 = b1 + 1;
		mn = min(mn,min(abs(i * i + b1 * b1 - d),abs(i * i + b2 * b2 - d)));
	}
	cout << mn;
	return 0;
}