3613.cpp

#include <iostream>
#include <string>
#include <algorithm>

using namespace std;

string str;
bool flag = false;

void JavaToC() {
    while (str.find('_') != string::npos) {
        int pos = str.find('_');
        str.erase(pos,1);
        str[pos] -= ('a' - 'A');
    }
}

void CToJava() {
    for (int i = 0; i < str.size(); i++) {
        if (str[i] < 'a') {
            str.insert(i, "_");
            str[i+1] += ('a' - 'A');
            i++;
        }
    }
}

int main(){
    cin >> str;
    if (str[0] < 'a' || str[0]=='_' || str[str.size()-1]=='_') flag = true;
    if (flag == false) {
        for (int i = 0; i < str.size()-1; i++) {
            if (str[i] == '_' && str[i + 1] == '_') {
                flag = true;
                break;
            }
        }
    }
    if (str.find('_') != string::npos) {
        for (int i = 0; i < str.size(); i++) {
            if ('A' <= str[i] && str[i] <= 'Z') {
                flag = true;
                break;
            }
        }
        JavaToC();
    }
    else {
        CToJava();
    }

    if (flag) cout << "Error!";
    else cout << str;
}