How do I edit my code so I can use size_t MyStrlen(const char *s1); instead of this?

Code:

#include 

int MyStrlen(const char *si);

int main() {
    char str[] = "a";
    char str1[] = "HELLO";
    char str2[] = "";
    
    printf("Length: %d \n", MyStrlen(str));
    printf("Length: %d \n", MyStrlen(str1));
    printf("Length: %d \n", MyStrlen(str2));
    return 0;
}

int MyStrlen(const char *si) {
    int input;  
    
    for(input = 0; *si; si++)
        input++;   
    
    return input;
}