Matrix product

#include <iostream>
#include <vector>

using namespace std;

typedef vector< vector<int> > Matrix;

Matrix product(const Matrix& a, const Matrix& b){
    Matrix s = a;
    int l = a.size();
    for(int i = 0; i < l; ++i){
        for(int j = 0; j < l; ++j){
            int sm = 0;
            for(int