Radio number of ladder graphs

Let G be a connected graph with diameter diam(G). The radio number for G, denoted by rn(G), is the smallest integer k such that there exists a function f: V(G)→{0, 1, 2,..., k} with the following satisfied for all vertices u and v:|f(u)-f(v)|≥diam (G)-dG(u, v)+1, where dG(u, v) is the distance between u and v in G. In this paper, we determine the radio number of ladder graphs.