\( t_1, t_2, t_3, ..., t_n \) are positive integers, \( S_n = t_1 + t_2 + t_3 + ... + t_n \), \( S_1 = 1^2 \), \( S_2 = 3^2 \), \( S_3 = 6^2 \), \( S_4 = 10^2 \), \( S_5 = 15^2 \) and similarly other terms are there. Following this pattern, if \( S_{10} = k^2 \) then \( k = \)