Write a function which calculates the position of
the n-th occurence of a
string sub in another string s.
If sub doesn't occur in s, -1 shall be returned.
def findnth(s, sub, n):
num = 0
start = -1
while num < n:
start = s.find(sub, start+1)
if start == -1: return -1
num += 1
return start
s = "abcxyzabcjkjkjkabclkjkjlkjabcjlj"
print(findnth(s,"abc", 4))