手芸王 | Codetter（こーどったー）

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#include <iostream>
#include <string>
using namespace std;
int check(const int n, const string str){
    if(n % 2 == 0) return -1;
    int p1 = n / 2, p2 = p1;
    for(int i = 0; i <= n / 2; ++i){
        switch(i % 3){
        case 0:
            if((str[p1] != 'b') || (str[p2] != 'b')) return -1;
            break;
        case 1:
            if((str[p1] != 'a') || (str[p2] != 'c')) return -1;
            break;
        case 2:
            if((str[p1] != 'c') || (str[p2] != 'a')) return -1;
            break;
        }
        --p1; ++p2;
    }
    return n / 2;
}
int main(){
    int n;
    string str;
    cin >> n >>str;
    cout << check(n, str) << "\n";
}
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