View Compilation Error

0_0_31477666_8378.cpp: In function 'int main()':
0_0_31477666_8378.cpp:6:39: error: 'x3' was not declared in this scope
    while(scanf("%d %d %d %d",&x1,&x2,&x3,&x4))
                                       ^
0_0_31477666_8378.cpp:6:43: error: 'x4' was not declared in this scope
    while(scanf("%d %d %d %d",&x1,&x2,&x3,&x4))
                                           ^