Structure padding in c

#include <stdio.h>
// #pragma pack(1) avoid strcture padding

// a _ _ _ | c c c c | d _ _ _ | b b b b => 16 bytes
struct s {
    char a; 
    int c;
    char d;
    int b;
}s1;

int main(void) {
    
    printf("%ld", sizeof(s1))  ;
    return 0;
}