function trickyFunction(num) {  
  let result = 0;  
  for (let i = 0; i < num; i++) {  
    result += i;  
    if (i % 2 === 0) {  
      result *= 2;  
    } else {  
      result -= 1;  
    }  
  }  
  return result;  
}  
console.log(trickyFunction(4));