var a = 49, b = 65;

var c = 100;

var d = 70;

function show() {

    if (a == 57) {

        a = 49;

    }


    var main = document.getElementById('txt1');

    var a1 = String.fromCharCode(a);

    var b1 = String.fromCharCode(b);

    var c1 = String.fromCharCode(c);

    var d1 = String.fromCharCode(d);


    main.value = a1 + b1 + c1 + d1;

    a = a + 1;

    b = b + 1;

    c = c + 1;

    d = d + 1;

}