Same idea in Swift times out in some of the test cases. Any optimization suggestions?

var t = Int(readLine()!)!

for _ in (0..<t) {
    var strArr = readLine()!.characters.map{String($0)}

    if strArr.count % 2 != 0 {
        print("-1")
    } else {
        var middle = strArr.count / 2
        var map = [String : Int]()

        for i in (0..<middle) {
            if map[strArr[i]] == nil {
                map[strArr[i]] = 1
            } else {
                map[strArr[i]] = map[strArr[i]]! + 1
            }

            if map[strArr[middle + i]] == nil {
                map[strArr[middle + i]] = -1
            } else {
                map[strArr[middle + i]] = map[strArr[middle + i]]! - 1
            }
        }

        var res = 0
        for (key, value) in map {
            res = res + abs(value)
        }

        print(String(res/2))
    }
}