Ошибка при повторном получении данных из потока cin

#include "Lib.h"
#include <conio.h>
 
void err(string s) //Catch errors
{
    throw runtime_error(s);
}
vector<int> input()
//Get from cin into vector integer numbers
{
    vector<int> v;
    int temp;
    cout<<"Please, enter row of integers, separated with space. To exit press <|>\n";
 
    while(cin>>temp) 
        v.push_back(temp);
    return v;
}
int get_sum(vector<int> values, int num)
//Count sum of <num> first elements from vector
{
    if (num>values.size())
        err("Vector size exceeded!\n"); 
    else if(num<=0)
        err("Wrong number of elements!\n"); 
    else { 
        int sum=0;
        for(unsigned int i=0;i<num;++i)
            sum+=values[i];
        return sum;
    }
}
int main() 
{
    //Loop while user wants to continue
    do
    {
        //get vector in input values
        vector<int> v=input();
        cout<<"Please, enter number of values to sum (from the first):\n";
        //get number of values 
        int num_sum=0;
        /*ERROR here. Seems like <cin> still contains something, 
        previous function didn't get, so it doesn't let new input*/
        cin>>num_sum;
        try {
            int sum=get_sum(v,num_sum);
            cout<<"Sum of "<<num_sum<<" elements is: "<<sum;
        }
        catch (runtime_error& e) {
            cerr<<"error: "<<e.what()<<endl;
        }
        catch(...) {
            cerr<<"Oups, unknown exception!\n";
        }
        //Prompt to continue
        cout<<"Press Y - for new input\n";
    }
    while(toupper(getch()) == 'Y');
    system("pause");
    return 0;
}