Roman numerals

[Wynd]

I am trying to make a program to convert from Roman numerals to base 10 numbers. I have the basic code, but the part I can't figure out is making numbers smaller if there is a smaller number in front of a larger one (example: I = 1, V = 5; I can do this but not IV which is 4). How can I do this? Here's what I have so far:

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string input;
    int total = 0;

    cout << "Enter the Roman numeral: ";
    getline(cin, input);

    for (int i = 0; i < input.length(); i++)
    {
        switch(input[i])
        {
        case 'M':
            total += 1000; break;
        case 'D':
            total += 500; break;
        case 'C':
            total += 100; break;
        case 'L':
            total += 50; break;
        case 'X':
            total += 10; break;
        case 'V':
            total += 5; break;
        case 'I':
            total += 1; break;
        }
    }

    cout << input << " equals " << total << endl;

    return 0;
}