fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)

fun readLargeInt () =
    valOf (TextIO.scanStream (LargeInt.scan StringCvt.DEC) TextIO.stdIn)


val () =
    let
        val n = readInt ()
        val cd_s = List.tabulate (n, fn _ => (readLargeInt (), readLargeInt ()))

        val mod_val = 1000000007
        val ans = List.foldl (fn ((c, d), acc) =>
                                 let
                                     val unit = (if c mod 2 = 0 then c div 2
                                                else c div 2 + 1) mod mod_val
                                     val d_mod = d mod mod_val
                                     val new_val = (unit * d_mod) mod mod_val
                                     val next_acc = (acc + new_val) mod mod_val
                                 in
                                     next_acc
                                 end)
                             0
                             cd_s
    in
        print (LargeInt.toString ans ^ "\n")
    end