Compilare: user.cpp:2:33: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type] std::ifstream f("aria.in");main(){long double N,a,b,x,y,X,Y,S=0;f>>N>>a>>b;x=a;for(y=b;--N;S+=x*Y-X*y,x=X,y=Y)f>>X>>Y;S+=x*b-a*y;freopen("aria.out","w",stdout);printf("%.5Lf",S/2);} ^ user.cpp: In function ‘int main()’: user.cpp:2:160: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] std::ifstream f("aria.in");main(){long double N,a,b,x,y,X,Y,S=0;f>>N>>a>>b;x=a;for(y=b;--N;S+=x*Y-X*y,x=X,y=Y)f>>X>>Y;S+=x*b-a*y;freopen("aria.out","w",stdout);printf("%.5Lf",S/2);} ^