The Return of the Roman Empire

#include<bits/stdc++.h>

using namespace std;

map<int,string>DecimalToRoman;

map<string,int>mp;

string s,R;

void DtoR(int value)
{
    DecimalToRoman[1000] = "M";
    DecimalToRoman[900] = "CM";

    DecimalToRoman[500] = "D";
    DecimalToRoman[400] = "CD";

    DecimalToRoman[100] = "C";
    DecimalToRoman[90] = "XC";

    DecimalToRoman[50] = "L";
    DecimalToRoman[40] = "XL";

    DecimalToRoman[10] = "X";
    DecimalToRoman[9] = "IX";

    DecimalToRoman[5] = "V";
    DecimalToRoman[4] = "IV";

    DecimalToRoman[1] = "I";

    int n = value;

    map<int,string>::reverse_iterator it;

    s = "";

    for(it = DecimalToRoman.rbegin(); it!=DecimalToRoman.rend(); it++)
    {
        while(value>=it->first)
        {
            s+=(string) it->second;

            value-=it->first;
        }
    }

    mp[s] = n;
}

void init()
{
    for(int i=1; i<4000; i++)
    {
        DtoR(i);
    }

    return ;
}

int main()
{
    init();

    while(cin>>R)
    {
        if (mp.count(R))
        {
            printf("%d\n", mp[R]);
        }
        else
        {
            puts("This is not a valid number");
        }
    }

    return 0;
}