#include <iostream>
#include <cmath>
#include <cstdlib>
 
int fac( int n )
{
    if( !n ) return 1;
    return n * fac(n-1);
}
 
double term( double a, int n )
{
    if( !(n % 2) ) a = -a;
    return a / fac(n);
}
 
int main ()
{
    int n;
    std::cout << "input sequence length" << std::endl;
    std::cin >> n;
 
    double a[n];
    double s = 0;
    for( int i=0; i<n; ++i )
    {
        std::cout << "input a[" << i+1 << "]";
        std::cin >> a[i];
        s += term( a[i], i+1 );
    }
    std::cout << s << std::endl;
    std::system("pause");
    return 0;
} 

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3 1.1 3.3 44.44