新手，写了个关于排列组合计算的程序，错了，请指教

#include <iostream>
using namespace std;

int main()
{
    int m, n;

start:
    do {
        cout << "请输入下标 m 和上标 n（n <= m)：";
        cin >> m >> n;
    } while (m < n);

    int a = 1, c = 1, i;
    for (i = 0; i < n; i++) {
        a *= m - i;
        c = c * (m-i) / (i+1);
    }

    cout << "请选择你想要输出的数据类型，输入a输出排列的结果，输入c输出组合的结果" << endl;
    char l;
    while (cin >> l) {
        if (l == 'a') {
            cout << "A(" << n << "," << m <<") = "
                << (int)a << endl;
            break;
        } else if (l == 'c') {
            cout << "C(" << n << "," << m <<") = "
                << (int)c << endl;
            break;
        }

        cout << "您输入的字母与提示字母不符，请核对后重新输入" << endl;
    }

    cout << "若需计算下个排列或组合，请输入r；否则退出。" << endl;
    cin >> l;
    if (l == 'r') goto start;

    return 0;
}