

function Main(input) {
  input = input.split("\n");
  var S = input[0].split("");
  var ans=0;
  for(var i=0;i<S.length;i++){
    if(i%2==0){
      if(S[i]==0){
        ans++;
      }
    }else{
      if(S[i]==1){
        ans++;
      }
    }
  }
  ans=Math.min(ans,S.length-ans);

  console.log('%s', ans);
  return ans;
}

function debug(n){
  var input = document.getElementById("input"+n).value;
  var ans=Main(input);
  var result="WA";
  if(ans==document.getElementById("output"+n).value.split("\n")[0]){
    result="AC";
  }
  document.getElementById("result"+n).innerHTML=result;
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));