Convert small letters to big and vice-versa

#include <iostream>
using namespace std;

int main() {

    char S[100];
    int i;

    cout << "Introduceti un sir: ";
    cin >> S;

    for (i = 0; S[i]; i++) {
        if ((S[i] >= 'a') && (S[i] <= 'z')) { S[i] = S[i] - 'a' + 'A'; }
        else if ((S[i] >= 'A') && (S[i] <= 'Z')) { S[i] = S[i] - 'A' + 'a'; }
    }

    cout << S << endl;

    cout << 'a' + 'a';

    system("pause");
}