function processData(input) {
    var distance = input.split("\n")[0].split(" ")[1];
    var numbers = input.split("\n")[1].split(" ");
    var totalCost = 0;
    for (var i = distance; i < numbers.length; i+=distance*2){
        totalCost += parseInt(numbers[i]);
    }
    console.log(totalCost)
} 

process.stdin.resume();
process.stdin.setEncoding("ascii");
_input = "";
process.stdin.on("data", function (input) {
    _input += input;
});

process.stdin.on("end", function () {
   processData(_input);
});