为啥字符串数组的长度为14

为啥数组的长度写14就对，写13就错，长度不就该是13吗？我数错了吗？

#include<stdio.h>
int main(){ 
    char s[14],c;int p = 0,j=1;
    scanf("%s",s);
     p = ((s[0]-'0')*1+(s[2]-'0')*2+(s[3]-'0')*3+(s[4]-'0')*4+(s[6]-'0')*5+(s[7]-'0')*6+(s[8]-'0')*7+(s[9]-'0')*8+(s[10]-'0')*9)%11;
    if(p == s[12]-'0'||(p==10&&s[12]=='X')){
        printf("Right");
    }else{
        if(p == 10){
            s[12] = 'X';
        }else{
            s[12] = p+'0';  
        }
        printf("%s",s);
    }
    return 0;
}