How can I get a rectangle with a straight border top right radius?

.box.n1,
.box.n2 {
  position: relative;
  width: 250px;
  height: 60px;
  margin: 10px;
}

.box.n1 {
  background: red;
}

.box.n2 {
  background: blue;
}

.box.n1::before,
.box.n2::before {
  position: absolute;
  content: '';
  width: 20px;
  height: 20px;
  background: #fff;
}

.box.n1::before {
  bottom: 0;
  left: 0;
  clip-path: polygon(100% 100%, 0 0, 0 100%);
}

.box.n2::before {
  clip-path: polygon(100% 0, 0 0, 100% 100%);
  top: 0;
  right: 0;
}

<div class="box n1"></div>
<div class="box n2"></div>