program easydiff
        implicit none
        include 'mpif.h'
        real u,v,dt,t,f1,f2,Time,w(2)
        integer ierror,istatus,nProc,myProc,tag,n,i
  
        call MPI_Init(ierror)
        call MPI_Comm_Rank(MPI_COMM_WORLD, myProc, ierror)
        call MPI_Comm_Size(MPI_COMM_WORLD, nProc, ierror)
  
        u=1
        Time=2
        n=100
        dt=Time/n
        t=0
        v=u
        do i=0, n-1
        if(myProc .eq. 0) then
        write(*,*)'U(',t,') = ', u
        endif
        if (i .eq. 0) then
        w(1)=u
        w(2)=v
        u=u+dt*(-u**2)
        w(1)=u
        endif
        if (i .gt. 0) then
C        write(*,*)'here',i,myProc
        if(myProc .eq. 0) then
        call MPI_Send(u,1,MPI_REAL,1,tag,MPI_COMM_WORLD,ierror)
        call MPI_Send(w,2,MPI_REAL,2,tag,MPI_COMM_WORLD,ierror)
        endif
        if(myProc .eq. 1) then
        call MPI_Recv(u,1,MPI_REAL,0,tag,MPI_COMM_WORLD,istatus,ierror)
        f1=-u**2
        call MPI_Send(f1,1,MPI_REAL,0,tag,MPI_COMM_WORLD,ierror)
        endif
        if(myProc .eq. 2) then
        call MPI_Recv(w,2,MPI_REAL,0,tag,MPI_COMM_WORLD,istatus,ierror)
        f2=-(2*w(1)-w(2))**2
        call MPI_Send(f2,1,MPI_REAL,0,tag,MPI_COMM_WORLD,ierror)
        endif
        if(myProc .eq. 0) then
        call MPI_Recv(f1,1,MPI_REAL,1,tag,MPI_COMM_WORLD,istatus,ierror)
        call MPI_Recv(f2,1,MPI_REAL,2,tag,MPI_COMM_WORLD,istatus,ierror)
        w(2)=u
        u=u+(dt/2)*(f1+f2)
        w(1)=u
        endif
        endif
        t=t+dt

        enddo
        call MPI_Finalize(ierror)

        stop
        end