codeforce - string transformation

#include<bits/stdc++.h>
using namespace std;
int main()
{
    string s;
    cin>>s;
    int len=s.size();
    char ch='a';
    for(int i=0;i<len;i++)
    {
        if(ch>=s[i])
        {
            s[i]=ch;
            ch++;

            if(ch>122)
                break;
        }
    }
    if(ch==123)cout<<s<<endl;
    else printf("-1\n");
    return 0;
}