package chelper;

import io.InputReader;
import io.OutputWriter;
import misc.SimpleSavingChelperSolution;


public class INVENTRY extends SimpleSavingChelperSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  @Override
  public void solve(int testNumber) {
    int n = in.nextInt();
    int[] spaces = new int[n + 2];
    int spn = 0;

    char[] chars = in.nextString().toCharArray();

    int t = 0;
    for (int i = 0; i < n; i++) {
      if (chars[i] == '#') {
        spaces[spn++] = t;
        t = 0;
      } else {
        t++;
      }
    }
    spaces[spn++] = t;

    int res = 0;

    for (int i = 0; i < spn - 1; ) {
      if (spaces[i] == 0) {
        i++;
        continue;
      }

      if (spaces[i + 1] >= 1) {
        res += spaces[i];
        spaces[i + 1] += spaces[i];
        spaces[i] = 0;
        continue;
      }

      // spaces[i + 1] == 0
      int need = 0;

      for (int j = i + 1; j < spn; j++) {
        if (spaces[j] == 0) {
          spaces[j] = 1;
          need++;
        }
        if (spaces[j] > 1) {
          int available = spaces[j] - 1;
          int take = Math.min(need, available);
          spaces[j] -= take;
          need -= take;
          if (need == 0) {
            break;
          }
        }
        res += need;
      }
      if (need != 0) {
        out.println(-1);
        return;
      }
    }
    out.println(res);
  }
}