start = 1;
end = 21;
speed = 2000;
s1 = 1;
s2 = 2;
s3 = 3;
s4 = 4;
s5 = 5;
s6 = 6;

function start_slideshow() {
timerID = setInterval('slideshow()', speed);
}

function slideshow() {
(s1 < 10) ? document.getElementById('s1').innerHTML = '<img src="slideshow/img-0' + s1 + '.png" alt="">' : document.getElementById('s1').innerHTML = '<img src="slideshow/img-' + s1 + '.png" alt="">';
(s2 < 10) ? document.getElementById('s2').innerHTML = '<img src="slideshow/img-0' + s2 + '.png" alt="">' : document.getElementById('s2').innerHTML = '<img src="slideshow/img-' + s2 + '.png" alt="">';
(s3 < 10) ? document.getElementById('s3').innerHTML = '<img src="slideshow/img-0' + s3 + '.png" alt="">' : document.getElementById('s3').innerHTML = '<img src="slideshow/img-' + s3 + '.png" alt="">';
(s4 < 10) ? document.getElementById('s4').innerHTML = '<img src="slideshow/img-0' + s4 + '.png" alt="">' : document.getElementById('s4').innerHTML = '<img src="slideshow/img-' + s4 + '.png" alt="">';
(s5 < 10) ? document.getElementById('s5').innerHTML = '<img src="slideshow/img-0' + s5 + '.png" alt="">' : document.getElementById('s5').innerHTML = '<img src="slideshow/img-' + s5 + '.png" alt="">';
(s6 < 10) ? document.getElementById('s6').innerHTML = '<img src="slideshow/img-0' + s6 + '.png" alt="">' : document.getElementById('s6').innerHTML = '<img src="slideshow/img-' + s6 + '.png" alt="">';

if (s1 < end-5)
{s1++; s2++; s3++; s4++; s5++; s6++; clearInterval(timerID); start_slideshow();}
else if (s1 == end-5)
{s1++; s2++; s3++; s4++; s5++; s6=1; clearInterval(timerID); start_slideshow();}
else if (s1 == end-4)
{s1++; s2++; s3++; s4++; s5=1; s6=2; clearInterval(timerID); start_slideshow();}
else if (s1 == end-3)
{s1++; s2++; s3++; s4=1; s5=2; s6=3; clearInterval(timerID); start_slideshow();}
else if (s1 == end-2)
{s1++; s2++; s3=1; s4=2; s5=3; s6=4; clearInterval(timerID); start_slideshow();}
else if (s1 == end-1)
{s1++; s2=1; s3=2; s4=3; s5=4; s6=5; clearInterval(timerID); start_slideshow();}
else if (s1 == end)
{s1=1; s2=2; s3=3; s4=4; s5=5; s6=6; clearInterval(timerID); start_slideshow();}
}