Please answer a question about the following article about New Delhi:

The Delhi Metro is a rapid transit system serving New Delhi, Delhi, Gurgaon, Faridabad, Noida, and Ghaziabad in the National Capital Region of India. Delhi Metro is the world's 12th largest metro system in terms of length. Delhi Metro was India's first modern public transportation system, which had revolutionised travel by providing a fast, reliable, safe, and comfortable means of transport. The network consists of six lines with a total length of 189.63 kilometres (117.83 miles) with 142 stations, of which 35 are underground, five are at-grade, and the remainder are elevated. All stations have escalators, elevators, and tactile tiles to guide the visually impaired from station entrances to trains. It has a combination of elevated, at-grade, and underground lines, and uses both broad gauge and standard gauge rolling stock. Four types of rolling stock are used: Mitsubishi-ROTEM Broad gauge, Bombardier MOVIA, Mitsubishi-ROTEM Standard gauge, and CAF Beasain Standard gauge.

How many of the Delhi Metro's stations are located underground?
35