Caesar Cypher (Cryptography)

#include <iostream>
#include <string>

using std::string;
using std::cout;
using std::endl;

const string chars = "abcdefghijklmnopqrstuvwxyz !?.";
const int n = 3;

string encode(string input)
{
    string output;
    int pos;
    for(int c = 0; c < input.length(); c++)
    {
        pos = chars.find( tolower(input[c]) );
        if(pos != string::npos)
        {
            output.append( chars.substr( ((pos + n) % 30), 1) );
        }
    }
    return output;
}

string decode(string input)
{
    string output;
    int pos;
    for(int c = 0; c < input.length(); c++)
    {
        pos = chars.find( tolower(input[c]) );
        if(pos != string::npos)
        {
            output.append( chars.substr( ((pos - n) % 30), 1) );
        }
    }
    return output;
}

int main()
{
    string message = "hello world";
    string encoded;

    cout << "Original -> " << message << endl;
    encoded = encode(message);
    cout << "Encoded -> " << encoded << endl;
    cout << "Decoded -> " << decode(encoded) << endl;

    return 1;
}