#include<stdio.h>
#include<gsl/gsl_cblas.h>
int main() {
    double A=1.0,X=1.0,Y=1.0;
    cblas_dgemv(CblasColMajor,CblasNoTrans,-1,1,0.0,&A,1,&X,1,0.0,&Y,1);
    printf("%lf\n",Y);
    return 0;
}