4. Jan's Notation

function solve(input){
    let arr = input.slice(0);
    let nums = [];

    while(arr.length>0){
        let operator = '';
        let result = 0;
        if(typeof(arr[0])=== 'number'){
            nums.push(arr.shift());
        }else{
            operator = arr.shift();
            switch(operator){
                case '+':
                result = nums[nums.length-2] + nums[nums.length-1];
                break;

                case '-':
                    result = nums[nums.length-2] - nums[nums.length-1];
                break;

                case '*':
                    result = nums[nums.length-2] * nums[nums.length-1];
                break;

                case '/':
                    result = nums[nums.length-2] / nums[nums.length-1];
                break;
            }
            nums.splice(-2,2,result);
        }
    }
    if(nums.length>1){
        console.log(`Error: too many operands!`);
    }else if(Number.isNaN(nums[0])){
        console.log(`Error: not enough operands!`);
    }else{
        console.log(nums[0]);
    }

}