View Compilation Error

0_0_1851824_28459.cpp: In function `int main()':
0_0_1851824_28459.cpp:7: error: `x1' undeclared (first use this function)
0_0_1851824_28459.cpp:7: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_1851824_28459.cpp:7: error: `x2' undeclared (first use this function)
0_0_1851824_28459.cpp:7: error: `y3' undeclared (first use this function)
0_0_1851824_28459.cpp:7: error: `y4' undeclared (first use this function)
0_0_1851824_28459.cpp:8: error: `y1' undeclared (first use this function)
0_0_1851824_28459.cpp:8: error: `y2' undeclared (first use this function)