igraph problem with verteces

guys my question is simple, why when I created a graph with edges like 3-5 and 5-7, the vertices are 7?

#U: is a vector with the name of vertex like 2-3-5-7 and list_edges is useful to check a condition that I supposed
edges.U <- function(U, list.edges){ 
  edges <- c()
  `%notin%` <- Negate(`%in%`) #define new operator
  for(row in 1:nrow(list.edges)){
    if( (list.edges[row, 1] %in% U) && (list.edges[row, 2] %notin% U) ){
      edges <- append(edges, c(list.edges[row, 1], list.edges[row, 2]) )
   #here I created correctly the edges vector, in this case I had 3-5 and 5-7
  subgraph.U <- graph(edges, U, directed=F) #####ERROR

In this case, why my vertices are wronged...?
How to set verteces with the function graph() by igraph?

kind regards, to the answer

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