age = getAge(numPeople-1)+2;这个不会，请问为什么要减一

#include <stdio.h> 

int getAge(numPeople)

{

    int age;

    if(numPeople==1)

    age=10;

    else

    age = getAge(numPeople-1)+2;

    return age;

}

int main() 

{

    int fifthAge = getAge(5);

printf("第5个人的年龄是%d岁", fifthAge); 

return 0;

}