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int find_substring(const string& s, const string& p) {
    int ns = s.size(), np = p.size();
    if(ns == 0 || np == 0 || ns < np) return -1;
    for(int i=0; i<ns-np; ++i) {
        int j = i, k = 0;
        while(k < np && p[k++] == s[j++]) {}
        if(k < np) continue;
        if(k == np) return i;
    }
    return -1;
}