* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    min-height: 100vh;
    background: rgb(199, 199, 199);
    display: flex;
    justify-content: center;
    align-items: center;
    font-family:'Trebuchet MS', 'Lucida Sans Unicode', 'Lucida Grande', 'Lucida Sans', Arial, sans-serif;
}
.calculator {
    width: 300px;
    height: 500px;
    box-shadow: 4px 4px 30px rgba(0,0,0,0.2);
    border-radius: 12px;
    background: #4c4c4c;
    overflow: hidden;
}

form input {
    width: 100%;
    height: 150px;
    border: none;
    border-radius: 12px;
    font-size: 2rem;
    padding: 1rem;
    color: #ffffff;
    background: #434343;
    text-align: right;
    pointer-events: none;
}
.button {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    padding: 20px;
}
button {
    flex: 0 0 22%;
    margin: 5px 0;
    border: none;
    width: 60px;
    height: 52px;
    font-size: 22px;
    font-weight: 600;
    border-radius: 5px;
    cursor: pointer;
}
.btn-yellow {
    background: rgb(239, 144, 55);
    color: #fff;
}
.btn-grey {
    background: rgb(224, 224, 224);
    color: #000;
}
.btn-equal {
    background: green;
    color: #fff;
}
.btn-clear {
    background: red;
    color: #fff;
}