View Compilation Error

0_0_30392266_21674.cpp: In function 'void convex_hull()':
0_0_30392266_21674.cpp:61:15: error: 't' was not declared in this scope
         while(t>=1&&Mul(s[i],stack[top],stack[top-1])>=0)  top--;//加入一个点 第三个点不满足，去掉前一个点前提 
               ^