AoC 2021 Day 12 Fortran

PROGRAM DAY12
    IMPLICIT NONE
    INTEGER :: I,J,K,N,M,IERR,START,END
    CHARACTER(LEN=5) :: NAME,A,B
    CHARACTER(LEN=5), ALLOCATABLE :: CAVENAMES(:)
    CHARACTER(LEN=8), ALLOCATABLE :: CONNECTIONS(:)
    LOGICAL, ALLOCATABLE :: VISITED(:)
    INTEGER, ALLOCATABLE :: CONN(:,:)

    OPEN(1,FILE="input.txt")
    N=0
    DO
        READ(1,*,IOSTAT=IERR)
        IF(IERR.NE.0) EXIT
        N=N+1
    END DO
    REWIND(1)
    ALLOCATE(CONNECTIONS(N),CAVENAMES(2*N),CONN(2*N,2*N),VISITED(2*N))
    READ (1,*) CONNECTIONS
    CLOSE(1)

    CAVENAMES=""
    CAVENAMES(1) = "start"
    CONN=0
    M=2
    DO I=1,N
        NAME = CONNECTIONS(I)(1:SCAN(CONNECTIONS(I),"-")-1)
        IF((.NOT.ANY(CAVENAMES.EQ.NAME)).AND.(TRIM(NAME).NE."end")) THEN
            CAVENAMES(M) = NAME
            M=M+1
        END IF
        NAME = CONNECTIONS(I)(SCAN(CONNECTIONS(I),"-")+1:LEN_TRIM(CONNECTIONS(I)))
        IF((.NOT.ANY(CAVENAMES.EQ.NAME)).AND.(TRIM(NAME).NE."end")) THEN
            CAVENAMES(M) = NAME
            M=M+1
        END IF
    END DO
    CAVENAMES(M)="end"
    DO I=1,N
        A = CONNECTIONS(I)(1:SCAN(CONNECTIONS(I),"-")-1)
        B = CONNECTIONS(I)(SCAN(CONNECTIONS(I),"-")+1:LEN_TRIM(CONNECTIONS(I)))
        J=MAXLOC(CAVENAMES,MASK=CAVENAMES.EQ.A,DIM=1)
        K=MAXLOC(CAVENAMES,MASK=CAVENAMES.EQ.B,DIM=1)
        CONN(MINLOC(CONN(:,J)),J) = K
        CONN(MINLOC(CONN(:,K)),K) = J
    END DO
    VISITED = .FALSE.
    WRITE(*,'(A,I0)') "Part 1: ", NPATHS(CONN,VISITED,1,M,1,.FALSE.)
    WRITE(*,'(A,I0)') "Part 2: ", NPATHS(CONN,VISITED,1,M,2,.FALSE.)
CONTAINS
    RECURSIVE FUNCTION NPATHS(CONN,V,A,END,PART,DOUBLE) RESULT(P1)
        INTEGER :: CONN(:,:),I,J,A,END,P1,PART
        LOGICAL :: V(:),VISITED(SIZE(VISITED)),DOUBLE

        VISITED = V
        VISITED(A)=.TRUE.
        P1=0
        DO I=1,SIZE(CONN,DIM=1)
            IF(CONN(I,A).EQ.1)CYCLE
            IF(CONN(I,A).EQ.0)EXIT
            IF(CONN(I,A).EQ.END) THEN
                P1=P1+1
                CYCLE
            ELSE
                J=IACHAR(CAVENAMES(CONN(I,A))(1:1))
                IF(J.GE.97 .AND. J.LE.122) THEN
                    IF(VISITED(CONN(I,A))) THEN
                        IF(DOUBLE .OR. PART.EQ.1) CYCLE
                        P1=P1+NPATHS(CONN,VISITED,CONN(I,A),END,PART,.TRUE.)
                        CYCLE
                    ENDIF
                END IF
                P1=P1+NPATHS(CONN,VISITED,CONN(I,A),END,PART,DOUBLE)
            END IF
        END DO
    END FUNCTION NPATHS
END PROGRAM DAY12