View Compilation Error

0_0_13831804_24648.cpp: In function 'int main()':
0_0_13831804_24648.cpp:62:51: error: 'abs' was not declared in this scope
    if(x>1 && !map[x-1][y] && abs(h[x-1][y]-h[x][y]) < v[x-1][y]) 
                                                   ^
0_0_13831804_24648.cpp:68:51: error: 'abs' was not declared in this scope
    if(x<n && !map[x+1][y] && abs(h[x+1][y]-h[x][y]) < v[x+1][y]) 
                                                   ^
0_0_13831804_24648.cpp:74:51: error: 'abs' was not declared in this scope
    if(y>1 && !map[x][y-1] && abs(h[x][y-1]-h[x][y]) < v[x][y-1]) 
                                                   ^
0_0_13831804_24648.cpp:80:51: error: 'abs' was not declared in this scope
    if(y<m && !map[x][y+1] && abs(h[x][y+1]-h[x][y]) < v[x][y+1]) 
                                                   ^