body {
  margin: 10;
}

.grid{
  display: grid;
  grid-template-columns: 1fr 1fr 1fr 1fr;
  grid-auto-rows:minmax(50px,auto);
  grid-gap: 10px;
}

#txtInput {
  grid-column: 1/5;
  grid-row: 1/3;
  background: black;
  color: white;
  font-size: 50px;
}
button {
  border-radius:10px;
  color: black;
}
.functionKey {
  background: #999999;
}
.enterKey {
  background: #ffffff;
}
#btn1 {
  grid-column: 1;
  grid-row: 5;
}
#btn2 {
  grid-column: 2;
  grid-row: 5;
}
#btn3 {
  grid-column: 3;
  grid-row: 5;
}
#btn4 {
  grid-column: 1;
  grid-row: 4;
}
#btn5 {
  grid-column: 2;
  grid-row: 4;
}
#btn6 {
  grid-column: 3;
  grid-row: 4;
}
#btn7 {
  grid-column: 1;
  grid-row: 3;
}
#btn8 {
  grid-column: 2;
  grid-row: 3;
}
#btn9 {
  grid-column: 3;
  grid-row: 3;
}
#btn0 {
  grid-column: 1;
  grid-row: 6;
}
#btnDec {
  grid-column: 2;
  grid-row: 6;
}
#btnEnter {
  grid-column: 3;
  grid-row: 6;
}
#btnAdd {
  grid-column: 4;
  grid-row: 3;
}
#btnSubtr {
  grid-column: 4;
  grid-row: 4;
}
#btnTimes {
  grid-column: 4;
  grid-row: 5;
}
#btnDiv {
  grid-column: 4;
  grid-row: 6;
}