@@ -120,6 +120,10 @@ int main(int argc,char* argv[]){

     f[0]= fopen(argv[1], "rb");

     f[1]= fopen(argv[2], "rb");

+    if(!f[0] || !f[1]){

+        fprintf(stderr, "couldnt open input files\n");

+        return -1;

+    }

     fseek(f[shift<0], shift < 0 ? -shift : shift, SEEK_SET);

     fseek(f[0],skip_bytes,SEEK_CUR);