/in/lab5.c: In function 'calculate':
/in/lab5.c:80:23: 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:236:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int l = 0; l < strlen(left); l++) {
                               ^