#include<iostream>
#include<windows.h>

using namespace std;

int main(){
    float r,d,C,S;
    string danwei = "cm";

    int CIN;
    cout <<"请输入选项(默认单位cm):"<<endl;
    cout <<"1.面积"<<endl;
    cout <<"2.周长"<<endl;
    cout <<"3.改单位"<<endl;

    cout <<">>>";
    cin >> CIN;

    if (CIN == 1)
    {
        cout <<"请输入半径(若是直径,请留空):";
        cin >>r;
        if (r==Unknown)
        {
            cout <<"请输入直径:";
            cin >>d;

            if (d==Unknown)
            {
                cout <<"无数据!";
                Sleep(2000);
                return 1;
            }
            r = d/2;
            S = r*r*3.14;
            cout <<"结果:"<<S<<danwei;
            Sleep(2500);
            return 0;                        
        }else
        {
            S = r*r*3.14;
            cout <<"结果是:"<<S<<danwei;
            Sleep(2500);
            return 0;
        }
               
    }else if (CIN == 2)
    {
        cout <<"请输入直径(若是半径,请留空):";
        cin >>d;

        if (d==Unknown)
        {
            cout <<"请输入半径:";
            cin >>r;

            if (r==Unknown)
            {
                cout <<"无数据!"<<endl;
                Sleep(2000);
                return 1;
            }else
            {
                d = r*2;
                C = 3.14*d;
                cout <<"结果是:"<<C<<danwei;
                Sleep(2500);
                return 0;
            }           
        }else
        {
            C = 3.14*d;
            cout <<"结果是:"<<C<<danwei<<endl;
            return 0;
        }
    }
}