how to get size of 2d vector in c++

#include <iostream>
#include <vector>

int main() {
    std::vector<std::vector<int>> matrix = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    int rows = matrix.size();
    int cols = matrix[0].size();

    std::cout << "Number of rows: " << rows << std::endl;
    std::cout << "Number of columns: " << cols << std::endl;

    return 0;
}