import java.util.*;
public class Main {
  private static final int LIM = 20;
  public static void main(String[] args) {
    final Scanner sc = new Scanner(System.in);
    while(sc.hasNext()) {
      final int r = sc.nextInt();
      final int n = sc.nextInt();
      if(r == 0 && n == 0)
        return;
      final int[] xl = new int[n];
      final int[] xr = new int[n];
      final int[] h  = new int[n];
      for(int i = 0; i < n; i++) {
        xl[i] = sc.nextInt();
        xr[i] = sc.nextInt();
        h[i]  = sc.nextInt();
      }
      final int[] hmax = new int[2 * LIM];
      for(int i = 0; i < n; i++)
        for(int j = xl[i]; j < xr[i]; j++)
          hmax[j+LIM] = Math.max(hmax[j+LIM], h[i]);
      for(int i = -LIM; i < LIM; i++)
          if(hmax[i+LIM] == Integer.MAX_VALUE)
            hmax[i+LIM] = 0;
      double ans = Double.POSITIVE_INFINITY;
      for(int i = -r; i < r; i++) {
        final int x = i < 0 ? ~i : i;
        ans = Math.min(ans, hmax[i+LIM] + r - Math.sqrt(r * r - x * x));
      }
      System.out.println(ans);
    }
  }
}