Adding a dash between 2 even numbers after printing out randomly generated numbers in array

So I've been trying to add a dash between every even number, but it's not outputting the dash. What am I doing wrong?

Generating 20 random numbers and putting them into array

var i = 0;
mainarr = [];

do {
    mainarr[i]=parseInt(Math.random()*10);
    i++;
}
while (i<20);

Printing the 20 numbers and adding dashes between every even number

var endVal =20;
for (i=0; i< endVal ;)
{
    main.innerHTML += mainarr[i];

        if ((i%2)==0) {
            i++;
            if ((i % 2)==0)
                main.innerHTML += "-";
        }
        else
            i++;
}