Three points \( O(0,0), P(a,a^{2}), Q(-b,b^{2}), a>0, b>0 \), are on the parabola \( y=x^{2} \). Let \( S_{1} \) be the area of the region bounded by the line \( PQ \) and the parabola, and \( S_{2} \) be the area of the triangle \( OPQ \). If the minimum value of \( \dfrac{S_{1}}{S_{2}} \) is \( \dfrac{m}{n} \), \( \gcd(m,n)=1 \), then \( m+n \) is equal to ________.
