var input = prompt ("Введите");
var output="";
var quantity = 1;
for (var i = 0; i < input.length; i++)
{
    if (i !== 0)
    {
        if (input[i-1] === input[i])
        {
            quantity++;
            if (i === input.length-1)
            {
                output+=input[i]+quantity;
                quantity = 1;
            }
        }
        else
        {
            if(quantity !== 1)
            {
                output += input[i-1]+quantity;
                quantity = 1;
            }
            else
                output += input[i-1];
            if (i === input.length-1)
                output += input[i];
        }
    }
    else
    {
        if (i === input.length - 1)
        {
            output = input[i];
            break;
        }
        else
            continue;
    }
}
alert(output);