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

using namespace std;

bool judge(const string &s) {
    string rs=s;
    reverse(rs.begin(),rs.end());
    int r=0,g=0,w=0;
    for ( char c: rs ) {
        switch ( c ) {
        case 'R':
            r++;
            break;
        case 'G':
            g++;
            if ( g>r ) {
                return false;
            }
            break;
        case 'W':
            if ( g==0 ) {
                return false;
            }
            if ( g>w ) {
                w++;
            }
            break;
        default:
            // NOT REACHED
            ;
        }
    }
    return r==g&&g==w;
}

int main(void) {
    string s;
    cin >> s;
    cout << ( judge(s)?"possible":"impossible" ) << endl;
}