この方法は誤差が多いな
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
ChangeWindowMode(TRUE);
SetGraphMode(640, 400, 32);
if (DxLib_Init() == -1) return -1;

int n = 0;
int naka = 0;

while (!ProcessMessage())
{
ClearDrawScreen();
DrawBox(0, 0, 399, 399, GetColor(255, 255, 255), TRUE);
DrawCircle(00, 399, 399, GetColor(0, 0, 255), true);

int x = GetRand(399);
int y = GetRand(399);
int Cr = GetPixel(x, y);

n++;
if (Cr == 255) naka++;
double kekka = (double)naka / n * 4.0;
DrawFormatString(400, 0, GetColor(255, 255, 0), L"%f\n", kekka);
ScreenFlip();
}
DxLib_End();
return 0;
}