Statement

The latest Trample's air raid against the United Nations of Antarctica (UNA) has destroyed everything! Now it's time for Edoardo, the emperor of the UNA, to plan the reconstruction of all the new N buildings of the continent. The i-th building will be constructed in a strategic position at coordinates (X_i, Y_i). Everyone knows that Trample's fighter jets are very powerful. In particular, a single airstrike is able to destroy all the buildings that have a given X or Y coordinate in a matter of seconds. Before giving his approval, Edoardo would like to know if the plan is resistant to air attacks. Can you help him find out the minimum number of airstrikes needed to destroy all the new buildings?