#include <stdio.h>
int main() {
char a[1001] = {0}, b[1001] = {0};
int len1 = 0, len2 = 0;
scanf("%s%s", a, b);
for (int i = 0; a[i] != '\0'; i++) {
len1++;
}
for (int i = 0; b[i] != '\0'; i++) {
len2++;
}
if (len1 < len2) {
for (int i = 0; i < len1; i++) {
if (a[i] != b[i]) {
printf("%d", i);
return 0;
}
}
printf("%d", len1);
return 0;
} else if (len1 >= len2) {
for (int i = 0; i <= len2; i++) {
if (a[i] != b[i]) {
printf("%d", i);
return 0;
}
}
printf("%d", len2);
return 0;
}
printf("%d", -1);
return 0;
}