.calc{
    width: 400px;
    height: 450px;
    background: gray;
    margin-left: auto;
    margin-right: auto;
    margin-top: 110px;
}
#screen{
    width: 80%;
    height: 40px;
    margin-left: 10%;
    margin-top: 10%;
    pointer-events: none;
    font-weight: bolder;
    font-size: 25px;
}
.btn{
    width: 60px;
    height: 45px;
    margin-left: 30px;
    background: bisque;
    margin-top: 30px;
    font-weight: bold;
    font-size: large;
}
.clear{
    width: 156px;
    height: 30px;
    margin-top: 10px;
    background: #3e3838;
    float: left;
}
.opr{
    background: #aaa2a2;
}