A shape with rounded corners and a triangle

Question

I need to make a shape like the picture below using CSS. There will be a number in the corner and content inside. Please tell me how to do it?

I was only able to do this:

.container {
  position: relative;
  width: 200px;
  height: 200px;
  background-color: #f2f2f2;
  border-top-right-radius: 80px;
  border-bottom-left-radius: 80px;
}

.background {
  position: relative;
  top: -20px;
  left: -20px;
  width: 0;
  height: 0;
  border-top: 100px solid red;
  border-right: 100px solid transparent;
}

.number {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 100%;
  height: 100%;
  color: #ffffff;
  font-size: 24px;
}

<div class="container">
  <div class="background">
    <span class="number">1</span>
  </div>
</div>

Answer 1

Try this out. Basically it's just a square with one rounded corner. Then the two angled pieces are the ::before and ::after elements rotated and manually placed. It's not perfect but it's close.

body{padding:2rem; background-color: #333;}

.container {
  position: relative;
  width: 200px;
  height: 200px;
  background-color: #FCFCFC;
  border-top-right-radius: 80px;
  border-bottom-left-radius: 80px;
}

.shape {
    position: relative;
    left: -20px;
    top: -20px;
    background: #b43b41;
    width: 100px;
    height: 100px;
    border-bottom-right-radius: 100%;
}

.shape::before,
.shape::after {
  display: block;
  content: ' ';
  background: #b43b41;
  width: 30px;
  height: 30px;
  transform: rotate(45deg);
  position: absolute;
  z-index: -1;
}

.shape::before {
  top: 6.5px;
    right: -14px;
}

.shape::after {
  bottom: -14px;
  left: 6.5px;
}

.number {
  position: absolute;
  top: -10px;
  left: -10px;
  color: #fff;
  font-size: 24px;
  width: 35px;
  text-align: center;
}

<div class="container">
  <div class="shape"></div>
  <div class="number">1</div>
</div>