function calc(expr) {
  expr = expr.split(' ');

  let stack = [];

  for (let token of expr) {
    if (token == '+' || token == '-' || token == '*' || token == '/') {
      let op2 = stack.pop();
      let op1 = stack.pop();
      let result;

      switch (token) {
        case '-':
          op2 = -op2;
        case '+':
          result = op1 + op2;
          break;
        case '/':
          op2 = 1 / op2;
        case '*':
          result = op1 * op2;
          break;
      }
      stack.push(result);

    } else {
      stack.push(Number(token));
    }
  }

  return stack.pop();
}