Problem with a run-length encoding program c++

I am trying to make a code that does run=length encoding based on the value entered by the user. Only accepts lower-case letters. Using two vectors. The problem is it doesn't read the last letter of the string. Example: eeeaaaad output: 3e4a (doesn't read d)

What could be the problem?

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

     bool is_small_letter(char c){
      return c >= 'a' && c <= 'z';
    }

    void runLength(string str, vector<char>& charachters, vector<int>& length) {
      int count;
      int i;

      for (i = 0; i < str.size() - 1; i++) {
        if (!is_small_letter(str.at(i))) {
          throw runtime_error("invalid input");
        }

        count = 1;
        while (str.at(i) == str.at(i + 1)) {
          count++, i++;
        }
        if (i == str.size() -1) {
        }
        charachters.push_back(str.at(i));
        length.push_back(count);
      }
    }

    int main() {
      string str;
      vector<char>charachters;
      vector<int>length;

      cout << "Enter the data to be compressed: ";
      cin >> str;
      try {
        runLength(str, charachters, length);

        cout << "The compressed data is: ";
        for (int i = 0; i <= length.size() -1; i++){
          cout << length.at(i) << charachters.at(i);
        }

      } catch (runtime_error &excpt) {
         cout << endl << "error: " << excpt.what();
      }

      return 0;
    }