let I() = stdin.ReadLine()
let N = int(I())
let f _ =
    let a = I().Split(' ')
    let c,D = double(a.[0]),int64(a.[1])
    let C = int64(c/2.+0.5)
    C*D
seq [1..N] |> Seq.map f |> Seq.sum
|> printfn "%d"