/in/lab5.c: In function 'calculate':
/in/lab5.c:75:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < strlen(input); i++) {
                       ^
/in/lab5.c: In function 'main':
/in/lab5.c:277:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int l = 0; l < strlen(left); l++) {
                               ^