$$d(x,y) = \lim_{n\to \infty} \sum_{j=0}^n \left(\frac{x_2-x_1}{n}\right)^{x_1+(j-1)\frac{x_2-x_1}{n}} + |f(x_1+j(\frac{x_2-x_1}{n}))-f(x_1+(j-1)(\frac{x_2-x_1}{n}))|^{x_1+(j-1)\frac{x_‌​2-x_1}{n}}$$