//#pragma GCC optimize("Ofast,no-stack-protector")
//#pragma GCC target("avx")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
using namespace std;

#define re return
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second
#define sqrt(x) sqrt(abs(x))
#define mp make_pair
#define pi (3.14159265358979323846264338327950288419716939937510)
#define fo(i, n) for(int i = 0; i < n; ++i)
#define ro(i, n) for(int i = n - 1; i >= 0; --i)
#define unique(v) v.resize(unique(all(v)) - v.begin())

template <class T> T abs (T x) { re x > 0 ? x : -x; }
template <class T> T sqr (T x) { re x * x; }
template <class T> T gcd (T a, T b) { re a ? gcd (b % a, a) : b; }
template <class T> int sgn (T x) { re x > 0 ? 1 : (x < 0 ? -1 : 0); }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int, int> ii;
typedef vector<ii> vii;
typedef vector<string> vs;
typedef double D;
typedef long double ld;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef vector<ll> vll;
typedef unsigned long long ull;
typedef tree <pair<int, char>, null_type, less<pair<int, char>>, rb_tree_tag, tree_order_statistics_node_update> _tree;

int main() {
    cout.precision(10);
    double r, a;
    cin >> a >> r;
    double h = 2 * sqrt(r * r - a * a / 4);
    if (r <= a / 2) cout << pi * r * r / 2 << endl;
    else if (r >= a * sqrt(2) / 2) cout << a * a << endl;
    else cout << pi * r * r + a * h - 2 * r * r * (double)asin(a * h / (2 * r * r)) << endl;
    re 0;
}

Look at this overkill and still Accepted
#include <math.h>
#include <iostream>
#include <math.h>
#include <iomanip>
using namespace std;
double n, r;
double pi = 3.141592654;
int main()
{
    cin >> n >> r;
    if(r <= n / 2)
    {
        cout << fixed << setprecision(3) << pi * r * r;
        return 0;
    }
    if(r >= n * sqrt(2) / 2)
    {
        cout << fixed << setprecision(3) << n * n;
        return 0;
    }
    cout << fixed << setprecision(3) << (r * (r * asin(-sqrt(r * r - n * n / 4) / r) + (-sqrt(r * r - n * n / 4) * -sqrt(-(-sqrt(r * r - n * n / 4) - r) * (-sqrt(r * r - n * n / 4) + r))) / r) / 2 -
                                         r * (r * asin(-n / 2 / r) + (-n / 2 * -sqrt(-(-n / 2 - r) * (-n / 2 + r))) / r) / 2 + n / 2 * sqrt(r * r - n * n / 4)) * 4;
    return 0;
}