Change characters in a string in C++

#include <iostream>
using namespace std ;
int main()
{
    string Num ;
    char a , b ;
    cout<<"Enter the number : ";
    cin>>Num;
    cout<<"Enter the digit you want to change : ";
    cin>>a;
    cout<<"Enter the digit to use instead : ";
    cin>>b;
    for(int i=0 ; i<Num.length() ; i++)
    {
        if(Num[i]==a)
        {
            Num[i]=b;
        }
    }
    cout<<Num;
}