Pyramid from Letters (C++)

#include <iostream>
#include <string>

using namespace std;

string getPyramid(const string&);

int main()
{
    string letters {""};
    cout << "Give me some letters: ";
    cin >> letters;
    cout << getPyramid(letters);
}

string getPyramid(const string &letters)
{
    string pyramid {""};

    for (int i {0}; i < (int)letters.length(); i++)
    {
        for (int j {0}; j < (int)letters.length() - 1 - i; j++) pyramid += ' '; //spaces
        for (int j {0}; j <= i; j++) pyramid += letters.at(j);  //begin to center
        for (int j {i-1}; j >= 0; j--) pyramid += letters.at(j); //center (exclusive) to begin
        pyramid += '\n';    //endline
    }

    return pyramid;
}