UT3USW

#include <iostream>
#include <random>

int main() {
    std::mt19937 g{std::random_device{}()};
    std::uniform_real_distribution<> d(0,1);
    int in = 0, N = 1e6;
    for (int i = 0; i < N; ++i) in += d(g)*d(g) < 1;
    std::cout << 4.0 * in / N << '\n';
}