function Main(input){
  input = input.split("\n");
  inputStr = input[0].split(" ");
  var N = Number.parseInt(inputStr[0]);
  //数値化して配列Aに放り込む
  inputStr = input[1].split(" ");
  var A = inputStr.map( inputStr => parseInt(inputStr));
  //数値化して配列Bに放り込む
  inputStr = input[2].split(" ");
  var B = inputStr.map( inputStr => parseInt(inputStr));
  //
  var cnt = 0;
  
  i=N-1;
  while(true){
    //i+1番目の足しこみとB[i]の消しこみ
    if(B[i] >= A[i+1]){
      cnt += A[i+1];
      B[i] = B[i] - A[i+1];
    } else {
      cnt += B[i];
      B[i] = 0;
    }
    //i番目の足しこみと消しこみ
    if(B[i] >= A[i]){
      cnt += A[i];
      B[i] = B[i] - A[i];
    } else {
      cnt += B[i];
      B[i] = 0;
    }
    i--;
    if(i<0){ break; }
  }
  //結果表示
  console.log(cnt);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
