#include  <stdio.h>

int strdiff(char* a, char* b){
    int index = 0;
    while(a[index] != '\0' && b[index] != '\0' && a[index] == b[index])
        index++;

    if(a[index] == '\0' && b[index] == '\0')
        return -1;
    char val = a[index]^b[index];
    int i = 0;
    while(val){
        if(val&1)
            return 8*index+i;
        val = val >> 1;
        i++;
    }
}

int main(int argc, char **argv)
{
        char s1[1000], s2[1000];
        gets(s1);
        gets(s2);
        printf("%d\n", strdiff(s1, s2));

        return 0;
}