INPUT "Enter the required starting digit of PI: "; D
    INPUT "Enter the number of digits wanted (max 10): "; C

    N = INT((D+20) * LOG(10) / LOG(2))
    acc = 0

    A = 3
    WHILE A <= 2*N

      M = INT(LOG(2*N) / LOG(A))
      P = A ^ M

      S = 0
      U = 1
      L = 1
      V = 0
      Q = 1
      R = 1

      FOR K = 1 TO N

        T = K
        IF Q >= A THEN
          DO
            T = INT(T/A)
            V = V-1
          LOOP UNTIL T MOD A
          Q = 0
        END IF

        Q = Q+1
        U = (U * T) MOD P

        T = 2*K - 1
        IF R >= A THEN
          IF R = A THEN
            DO
              T = INT(T/A)
              V = V+1
            LOOP UNTIL T MOD A
          END IF
          R = R-A
        END IF

        L = (L * T) MOD P
        R = R+2

        IF V > 0 THEN
          T = inv.mod(L, P)
          T = (T * U) MOD P
          T = (T * K) MOD P

          FOR I = V TO M-1
            T = (T * A) MOD P
          NEXT

          S = S+T
          IF S >= P THEN S = S-P
        END IF

      NEXT K

      T = pow.mod(10, D-1, P)
      S = (S * T) MOD P
      acc = acc + S / P
      acc = acc - INT(acc)

      A = next.prime(A)
      SCAN
    WEND

    PRINT "Decimal digits of PI at position "; D; ": "; STR$(INT(acc * 10^C))
    END

FUNCTION next.prime(N)
    DO
      N = N+1
    LOOP UNTIL is.prime(N)
    next.prime = N
END FUNCTION

FUNCTION is.prime(N)
    IF N MOD 2 = 0 THEN is.prime = 0 : EXIT FUNCTION
    FOR I = 3 TO SQR(N) STEP 2
      IF N MOD I = 0 THEN is.prime = 0 : EXIT FUNCTION
    NEXT
    is.prime = 1
END FUNCTION

FUNCTION pow.mod(A, B, M)
    R = 1
    DO
      IF B AND 1 THEN R = (R * A) MOD M
      B = INT(B/2)
      IF B = 0 THEN EXIT DO
      A = (A * A) MOD M
    LOOP UNTIL 0
    pow.mod = R
END FUNCTION

FUNCTION inv.mod(X, Y)
    V = Y
    C = 1
    DO
      Q = INT(V / X)

      T = C
      C = A-Q*C
      A = T

      T = X
      X = V-Q*X
      V = T
    LOOP UNTIL X = 0
    A = A MOD Y
    IF A<0 THEN A = A+Y
    inv.mod = A
END FUNCTION