To find the minimum period of a string

If a string is repeated multiple times with a string of length k, we say that the string is in the period of K, for example: Abcabcabcabc with a period of 3, (note that it also takes 6 and 12 as periods), enter a string of not more than 80 in length, and output a minimum period.

The idea is to enumerate the cycles from small to large, and meet the criteria to jump out.

#include <cstdio>
#include <cstring>

int main ()
{
    //freopen ("A.txt", "R", stdin);
    Char s[100];
    int flag;
    while (scanf ("%s", s)!=eof)
    {
        int len=strlen (s);
        for (int i=1;i<len;i++)
        {
            if (len%i==0)  //Enumeration I is period
            {
                flag=1;
                for (int j=i;j<len;j++)//Compare whether the current character is equal to the corresponding character in the first period,
                {
                    if (s[j]!=s[j%i]) {flag=0;break;}}
            }
            if (flag) {printf ("%d\n", I); break;}
        }
    }
	return 0;
}