function processData(input) {
  var lines = input.split('\n');
  var line = lines[0].split(" ");
  var n = parseInt(line[0], 10);
  var k = parseInt(line[1], 10);
  var c = lines[1].split(' ').map(function (a) { return parseInt(a, 10); });
  var sum = 0;
  var sumin;

  for(var i = 0; i < n; i++) {
      //console.error("i = " + i);
      sum += c[i];
      i += 2*k;  
  }
  sumin = sum;
   //console.error("sumin = " + sumin);
    
for(var j = 1; j <= k; j++){    
  sum = 0;
  for(var i = j; i < n; i++) {
      //console.error("i = " + i);
      sum += c[i];
      i += 2*k;  
  }
  if (sum < sumin) sumin = sum;
  //console.error("sum = " + sum);
  
}
    
    
    console.log(sumin);
    
} 

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

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