body {
  margin: 0;
  padding: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

.cell {
  width: 150px;
  height: 150px;
  font: bold 20px roboto;
  box-sizing: border-box;
  padding: 25px;
  color: #000;
  background-color: #ffcccc;
  transition: background-color 250ms linear;
}

.cell:hover {
  background-color: #ff6666;
  cursor: pointer;
}

.c1 {
  border-radius: 180% 0 0 0;
  vertical-align: bottom;
  text-align: right;
}
.c2 {
  border-radius: 0 180% 0 0;
  vertical-align: bottom;
}
.c3 {
  border-radius: 0 0 0 180%;
  vertical-align: top;
  text-align: right;
}
.c4 {
  border-radius: 0 0 180% 0;
  vertical-align: top;
}