#include <iostream>
#include <vector>
#include <string>
using namespace std;

int main() {
    int n;
    string s;
    cin >> n >> s;
    vector<int> preM(n + 1, 0), preY(n + 1, 0);
    for (int i = 1; i <= n; ++i) {
        preM[i] = preM[i - 1] + (s[i - 1] == 'M');
        preY[i] = preY[i - 1] + (s[i - 1] == 'Y');
    }
    int totM = preM[n];
    int totY = preY[n];
    int m = (n + 1) / 2;
    for (int k = m; k < n; ++k) {
        int mM = preM[k];
        int fM = totM - mM;
        if (mM == fM) continue;
        int mY = preY[k];
        int fY = totY - mY;
        if (mY == fY) continue;
        cout << k << endl;
        return 0;
    }
    cout << -1 << endl;
    return 0;
}