function subtraction(arr)   
 {  
  if (Object.prototype.toString.call(arr) === '[object Array]') {  
    var total = arr[0];  
    if (typeof (total) !== 'number') {  
      return false;  
    }  
    for (var i = 1, length = arr.length; i < length; i++)  
    {  
      if (typeof (arr[i]) === 'number')  
      {  
        total -= arr[i];  
      }   
      else   
      return false;  
    }  
    return total;  
   }   
    else  
     return false;  
  }  
console.log(subtraction([7,3, 2,-1])); 